Sabaton is a lot of fun, and they're certainly more historically-accurate than, say, Led Zeppelin. That said, it's worth remembering they're ultimately just a band and a lot of what they say is more 'inspired' by history than reflective of it.

There's a reason I mention that, which I'll return to in a bit. But for the moment, I can try to address a few of the points you've asked about.

The simple answer is to your first question. Not only do soldiers have a chance to return home, the vast majority of them did. The 30 Years War was a calamity for humanity, and estimates usually put the number of military casualties in the 200-300,000 range. There isn't a reliable way to source how many of those casualties were fatalities, and it's important to remember the same soldier can be a casualty multiple times. In fact, analysis of mass graves from battle sites reflects exactly that: http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0178252

Here, we see that a high percentage of the dead had signs of previous injuries. This is just one grave site, but there's no reason to assume it's in some way exceptional--it's from neither aparticularly early nor particularly late battle in the war, though it was a seminal moment in one of the war's 4 phases.

And then there is the sheer number of soldiers involved. Estimates vary wildly, and given the fluctuations across the 30 years of warfare, it's hard to pin down how many soldiers fought in the war in a truly meaningful sense. Some Scottish volunteers might have spent years in the war, while some mercenaries might have only served for a single engagement before dispersing, returning home, or turning to banditry.

Given that, we can still calculate the involvement of at least a million individual soldiers in the war, and likely far more than that. The majority of them would expect to return home.

But you also asked about the Swedish Intervention, mentioning how they were basically sent to their certain death. You're correct that Gustavus Adolphus did mandate a nationwide levy; not even noble families were exempt from his draft. But I'm not sure "certain death" is entirely accurate either. Adolphus' forces were some of the best-commanded and equipped in the war, and in fact while his invasion did suffer some setbacks, they were almost undefeated in major engagements. Adolphus himself would die in battle, but this was the beginning of a century of Swedish domination of northern Europe. That's not to say their army didn't suffer horrible losses--they did. But in general, Swedish soldiers were on the winning side of most battles and had a lower mortality rate than their opponents.

I can't speak specifically to letters or desertion, but I do have a brief addendum: There is one group for whom your description of the war as a meat grinder is apt, and it's not really the Imperial nor Protestant(ish) side. They are the more or less innocent people of the German states, most of whom had nothing to gain from the wars nor the rivalries of various monarchs they'd never so much as see. The 30 Years War is famously bloody,but not so much for the soldier's suffering as the suffering they inflicted on the civilians. Soldiers, often but not always mercenaries, devastated the German states. Entire villages and towns were razed; murder and pillaging was not just common but the rule as armies 'foraged' through the countryside. In recent years some scholars have tried to push back against this, arguing that the war's brutality is somewhat overblown and only exacerbated preexisting conditions, like urban disease and malnutrition. That may be true; but in general, the academic consensus is that the war saw a mortality rate of at least 15 to 20 percent of the entire population of the German states. That's not counting people displaced--probably twice that number--or maimed, or robbed of their livelihoods, or raped, or otherwise maltreated by the waves of foreign armies. It's widely held that it took the better part of a century for most of the affected towns and cities to recover from the depopulation and destruction wrought by the war.

So while I might sympathize with a Swedish soldier, levied by a king he didn't know to fight a war against people he'd never met over an empire he'd not be in any position to benefit from (The war stopped being principally religious very quickly), I think that even greater sympathy should probably lie with the various families who saw their towns, farms, and their entire country, sundered by violence. Millions died; estimates range from a minimum of 4 to as high as 12 million people. Again, these are nearly all civilians. At the most, five percent or so were likely soldiers.

Sabaton, in short, isn't wrong. Soldiers did have it rough. But in general, they should expect to survive the war, and those who did would frequently have enough rape and loot that the invasion was seen as a high point in Swedish history for the following centuries. The German citizens who starved en masse had a much, much higher mortality rate.