"Not all Veterans are wholly sane, or even wholly men any more. Many suffer from severe battle psychosis to the extent that they hunger for battle, while others are haunted, paranoid individuals who believe that somewhere out there is the bullet or las bolt destined for them. The more dubious Imperial Commanders do not think twice about subjecting their Guardsmen to atrocities which would be considered barbaric by many, such as forced addiction to certain combat drugs, or the implantation of adrenal and endochrinal glands that turn the Veteran into a frenzied killing machine. In the most horrendous instances, the Veteran may have undergone such traumas that they are completely unhinged and unsuitable for normal service - tales abound of platoons forced to eat their dead comrades or starve; of being isolated for years at a time under continuous shelling until they are half-blind and deaf; of seeing alien and Chaotic monstrosities so hideous that they defy sanity. [...]"-- Inquisitor : Imperial Guard VeteransThis is something I hadn't considered, but would mean that alot of war torn worlds would need mental institutes constructed after, to house all the crazy lunatics and guardsmen with horrific withdrawal symptoms that have been turned loose into their relatively vulnerable society.
On the other hand, the issues of shellshock and PTSD are known to be surprisingly uncommon in societies that encompasses large amounts of people that should have PTSD or PTSD-like issues. Exactly why that is, I'm not sure, but it could be theorized that it has something to do with people being able to relate to eachother and the experiences.
After all, the main issue of modern-day soldiers, primarily in the western world, is an inability to readjust and relate to civilian life after coming home, facing people that cannot possibly understand, unable to convey their thoughts and emotions. This is exacerbated by the sheer hypocrisy of modernist culture and cultural marxism, teaching doctrines that mesh incredibly badly with human nature and factual reality, and the reality a lot of soldiers have experienced first hand.
In a society with a lot of veterans and a pre-existing focus on survival and discipline, with a dogma maintaining hierarchy and teaching that the world is in fact dangerous - not uncommon in the Imperium, to say the least - I'd expect that the issues you mention would be greatly reduced.
But that's mainly just musings on my own part. Armchair sociology and psychology, at best. I just wanted to raise the point.
Also, the truly horrific horrors of the 41st millennium are not faced by the average guardsman. They are faced by guardsmen, yes, and those guardsman may be average, yes, but the vast majority of the Imperial Guard likely do not have to face Chaos or Tyranids specifically, and while there certainly are regular "horrors of war" going on, we're not talking reality-bending insanity.
I always had the feeling that no-one in the Imperial Guard that faces Chaos, Necrons or Tyranids lives to tell the tale, anyway. At least not for long.
Though bare in mind that the adjectives used in the veteran description do indicate that these are some of the more extreme examples rather than the norm (not the battle psychosis but the eating the dead etc).
I do remember reading that oft times a Guard Regiment isn't sent outside of their sector or even their sub sector except in the case of the largest war zones. Depending on the conflict it might be practical for a triumphant Imperial Guard regiment to return home as a propaganda base for further recruitment.
It can take decades for the Munitorium to organise a full blown crusade or campaign which consists of dozens or even hudnreds of regiments, Marine Chapters, Mechanicus experts etc. I could see that part of this process of recruitment might include a period of Guard Triumphs (in the roman style) http://en.wikipedia.org/wiki/Roman_triumph to get the local population enthusiastic about joining up.
So for example war is brewing in some sector vs Death Guard or some other horrible foe. High Lords declare they need an additional 100 regiments raised from scratch. Department Munitorium know that under normal circumstances they can raise 20 regiments.
So maybe instead they instigate a few brushfire wars and relatively minor skirmishes against known pirate bases, minor rebellious worlds that they have been putting off dealing with and the like.
After these easy conflicts are resolved a half dozen regiments are retired and the members transported back home for a heroes welcome with stories of glorious victory and good pay.
Ten or twenty years later when the Guard comes to receruit there are plenty of young men and women eager to join up to live up to the exploits of Uncle Charlie not realising that they are at the bum end of the Department Munitoriums galactic recruitment cycle and are being shipped off to a real hell hole with no prospect of returning home.
In other words retirment and transportation back home might be seen as an investment on the part of the Munitorium.
On the issue of "returning home", I can see several problems. But at the same time, it's entirely reasonable that it shouldn't be an enormous problem for a decorated or retired guardsman to "planethop". After all, there are very few worlds in the Imperium (relatively speaking) that are truly isolated. Even backwater worlds are tithed regularly.
Let's say it's a guardsman that was part of a regiment that was just retired. For whatever reason, they're in the Sector Cadia. Let's say, the regiment, or what remains of it, is originally from.. I don't know.. let's say.. Landunder. The weird planet in Sector Calixis, Malfian Sub-Sector.
Is it really so outlandish to think that they would hitch a ride to Scintilla, then to Malfi, and then try to find transportation to Landunder? Going to any Agri-World, something Fleet or Navy ships have to do all the time, I imagine, often gives you the chance to go to a great many Hive Worlds. And Hive Worlds are in turn enormous hubs of trade, commerce and production.
Traversing the Imperium this way is likely a slow process, and they would likely have to work while doing so, but I don't see why it would be so strange. The average ship has possibly tens of thousands of crewmen. Imperial Navy ships are built to transport entire regiments to warzones. When they leave, why would they leave empty unless they have to? The average ship resupplies people on nearly every port, to some degree or another, and many ships have countless unskilled workers, perhaps even press-ganged - congratulations, for this stint, we've got our friends in the Imperial Guard helping us out.
And woe to the pirates that attempt to attack a ship carrying veterans going home to mom and pop.
I realize that the universe is meant to be grimdark, but at the same time, let's not make it grimderp. It's dark because that's where the universe is at. It's grim because that's what it needs to be to survive. That doesn't mean that everything is horrifying or depressing because reasons.
Save for penal regiments and the like, there has to be reasons people sign up for the guard, even beyond the indoctrination. There has to be a reason there's not massive defections going on throughout all echelons of the command structures. I cannot imagine that Imperial Guardsmen are not paid - usually - and that their families are not cared for - usually - or that they can't retire after a tour of duty.. less usually.