I know this seems like such a technical detail but I know that a huge part of 1914 was the "Race to the Sea" in which armies tried to outflank each other, and when they reached the coast there was no more room. Except, when I thought about it, there had to have been at least some room, even if just dozens or hundreds of feet; it would seem ridiculous for soldiers to keep digging all the way into the beaches and only stop once waves were crashing over their heads.
Likewise, and more generally, how close could the edges of frontlines get to the borders of neutral countries? If my house was on the border itself, would I be able to look out my window and see soldiers just outside or would there be a "courtesy zone" of a few miles to prevent diplomatic incidents (i.e. like soldiers accidentally invading a neutral country because they got lost, or stray artillery shell falling on neutral territory)?
Hello, so you may be interested in a comment I made recently relating the south end of the front here post by myself u/jonewer and answers several of your points.
To the north, near Nieuport, the fortifications did indeed go all the to the waterline as you can see here with a schematic of the German trenches as they would have been in June 1917 shown in red on this map
TL;DR the front line went quite literally from the sea to quite literally the Swiss border, if you lived on the border, you probably wouldn't want to spend too much time looking out of your window because lots of men were pointing guns at each other.