I'm going to answer some of your follow up questions first because they are a bit more straightforward and can hopefully give some context for your main question.

How common would machine guns be along the lines?

This is a bit of a tough question to answer because it comes down to it depends, mainly, where we're talking about. We can generally reduce the Western Front to "quiet zones" and "loud zones", 'quiet' being where the trenches had a significant distance (600-800 meters at times) between them and immediate assault was not a daily issue. These obviously had less entrenched positions and machine gun placements than loud, or 'hot', zones where the trenches could at times be within earshot of each other.

We can speak generally about organization however and derive some truth from that. The heart of the German army was the combat division. It had two brigades, each with two regiments of 3000 men each. These regiments would contain three battalions of 1000 men; and each battalion had four companies of 250. The divisional commander, with control of roughly 12,000 men total, had an artillery brigade of 72 guns and an additional company with their 1908 Maxim machine guns and an additional 18 105mm howitzers.

We see similar numbers and organization with the French. Each brigade would be supplied with a section of machine gunners using a Model 1907 Saint-Etienne.

So to answer your question of 'how common are machine guns along the line', common enough. I don't want to give a ratio to distance because it would be pulled from my ass and I don't think a single historian can give you anything but a crap shoot. What we can say though is that if you were approaching an enemy trench you would be in the firing line of at least one or two machine guns and likely more if you were in a real hot area.

Would soldiers commonly have used craters as cover (with the support of their officers) or was that generally only done when the assault had begun to falter?

This will lead me nicely into your main question. Yes, soldiers used cover. No, soldiers were not idiots. We only have historical basis to say men walked in the traditional shoulder to shoulder form early on in the war, as in, August and early September of 1914. This was the early learning period as even in the Franco-Prussian War these tactics were at least partially useful.

Throughout '14 and '15 and even into '16 though men were still kept in something called close order. This has a reason that is beyond "they were stupid and couldn't learn" -- these were untrained conscripts they were sending into battle. Against machine guns and entrenched positions. The fact is, conscripted and non-professional soldiers are not reliable and need to be kept on a close leash. It's part of why line tactics were so useful for basically all of human history -- it keeps all the men within earshot of the officer and by staying relatively close together they build each others confidence.

Clearly they would not stand in tightly packed formations anymore but they would approach enemy trenches as a unit rather than a roaming mass just kind of spread out and doing their own thing. They also would at times be covering large distances -- like I said, the distances reached hundreds of meters at times. They would walk initially and when it got hot, they would run and duck and dive and dodge but most importantly they would have to keep moving.

Would an attacking soldier only really have seem combat once he was inside the enemy trenches or would they have been able to effectively engage while still crossing no-mans land?

This kind of touches on something else I really want to emphasize -- most combat in WWI was not the large scale attacks you imagine. We have these big, "epic" battles like Passchendaele, the Somme, Verdun, the 2nd Battle of Ypres, etc. of men swarming enemy trenches and being cut down by machine guns en masse and so forth. While these persisted for a long time in their regions it was not a front-wide offensive. Generally everywhere besides these major offensives would be relatively quiet as both sides were concentrating their efforts on that one area. Even in these areas it's not like men were getting up every single day for the entire 6-8 month duration of them and charging en masse every day. It would be a cycle of artillery bombardment, assaults, retreats, rests, harassment, etc. Many times a company would successfully take part of an enemy trench, realize they can't hold it, and just retreat back their trench in the same day and then be cycled out a few days later and go on with their lives. In fact we have plenty of accounts of men who survived the Somme and Verdun and such from both sides who survived an attack on an enemy trench, retreated back, and lived the duration of the war.

A significant portion of the time a soldier would be sitting around doing nothing. The most active parts of a soldiers day was dusk and dawn when they would "stand to", basically be at the ready for an enemy offensive that may come as those were the times that were seen best to attack. The rest of the night would be spent reinforcing trenches (if you did it during the day enemy aviation could spot your movements) and then the day was basically lounging around.

Then when we get to the moderate amount of combat soldiers saw it was not as you imagine. Most of what a soldiers combat duties would be would be raids and patrols, not major offensives on enemy trenches. It would not be uncommon for 5-150 men (yes, even as little as 5!) to raid an enemy trench in the dead of night and cause a havoc, take some prisoners and documents, and go back to their trench for interrogation and rest and so forth. Men were also sent out into observation posts in no mans land, basically glorified pot holes, to listen in on the enemy and scout out their patrols. Your other combat role would be either protecting worker parties (soldiers on "rest" who were repairing and laying down barbed wire and were therefore exposed) from the trench or going on an offensive patrol to harass the enemy worker parties.

With that said, it is said that most men in the French army served in Verdun. I know I'm really getting tangential here but I still think it's cool. The French and Germans had many differences and how they cycled out troops is one of them. What the French liked was to cycle their men out between fronts -- that is, you'd serve over in the Ypres region for a while, then in Alsace, then in Verdun, then there and here and over there again. You basically were cycled out across the entire Western Front if you managed to live so it's kind of a little Snapple Fact that most French soldiers would be involved in Verdun in one way or another since it lasted for so many months and so many men cycled through there. The Germans on the other hand, if you're curious, opted to cycle men between trench lines. So you'd serve on the front trench this week, then in the communication trench this week, and the reserve trench the next, or whatever the time frame is for that particular zone. Basically having your immediate risk from the enemy being reduced each "cycle" and then being sent back to the front to start over.

Anyway,

Given that (in my understanding) troops were expected to walk in a formation for much of the distance between the two lines of trenches would the presence of a machine gun firing at them generally have ensured that none would make it to the enemy's trench? How effective was artillery generally at clearing machine guns out?

This is a lesson that was learned brutally at the Somme. Mainly, that last sentence -- how effective was artillery at clearing machine guns out? Officers felt it could do a damn good job and in 1916 when the British launched an extended artillery barrage lasting for days on a small targeted area of the German trench system they expected all the barbed wire and all the machine guns to be destroyed. I mean seriously -- we're talking about hundreds of thousands of massive artillery shells hitting one little area of the front for days on end. How could it not be turned to rubble was the thought.

Well, they weren't. The barbed wire was in tatters in some places but in tact in others. The men just hunkered down in concrete and more fortified regions and when the artillery stopped and the British came running they just came out of their holes and manned their guns and the British were essentially attacking the same trench that was there a few days ago except now the terrain was muddy, covered in craters and muck and debris that made it difficult to walk in, and those same damn machine guns were still shooting at them. So yeah, they would lose about 60,000 men in the first day of fighting.

Men would reach the trenches semi-frequently in major attacks though, despite all I said, just taking significant losses on the way there and thus not being able to hold it. Trenches would change sides semi-frequently in the war from just regular attacks and during major offensives. What would happen is that once the front line trench would be breached both sides would be keen on just letting the enemy have it. Then they would retreat, gather their forces, and perform a counter attack and retake the trench with reinforced numbers and before the enemy can reinforce themselves. They'd retake the trench after a few minutes, hours, or even days and things would be back to square one. You could never really secure a trench after taking it was the issue because you were always inherently undersupplied and undermanned compared to your enemy which had their entire infrastructural system at their backs.

Fighting in the trenches was generally comprised of pistols, knives, blunt objects, brass knuckles, and bayonets. Both in major offensives and in night time raids. As you asked earlier yes, men would fire on enemy trenches to try and suppress them but it's not like combat was fought between men standing in the open and men sitting behind sandbags -- it just wouldn't work. You had to get in the trench to fight the enemy, that was really the problem.

A quick side note is that this, not being able to resupply men after securing a trench safely, and that thing I said earlier about only attacking one area of the front with all your force at once, would be two major things that would change by 1918 and allow trenches to become glorified stepping stones in the wake of said advancements/changes. I kind of want to go on but I'd rather not fill up this post any more because I feel 1918 warfare wasn't really what you are asking as it was a totally different beast from '15 and '16 warfare :P

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Notes:

The Marne 1914: The Opening of the Great War by Holger Herwig

French Strategy and Operations in the Great War by Robert Doughty

Life and Death in the Trenches in World War I by Andy Simpson

The First Day on the Somme by Martin Middlebrook