One of the most prohibiting factors here is the distance between the planets, and that the Imperium isn't actually well-connected when you think how space travel works - or even consider the huge gaps in unknown space
between the patches of Imperial systems, because no stable warp corridor means no space travel.
The cost
and time to get someone from A to B surely are important factors when determining what the Imperial Guard can and cannot do. Yes, maybe planet Randomworld has a good way of treating PTSD - lucky PDF! That doesn't really help the guys that were conscripted by the Munitorum, though, considering that they're never going to see their home again.
It is also worth considering that the Imperial Guard, from all we're told in the codices, seems to operate much more like a Napoleonic Era army rather than a "modern" military force. There's no logistics regiments, just semi-civilian baggage trains that sometimes include the soldiers' children sired during the campaign. Every Guard regiment raised on a planet is a fighting unit, intended to either battle alone or be combined with other regiments of useful types into army groups. Supply drops are handled by the Navy.
This is probably why medical care for the Imperial Guard is largely concentrated in the field hospitals of the Orders Hospitaller - although Segmentum Command
does have a medical department which coordinates with the various regiments' own medics by distributing valuable information.
Ultimately, this leads me to believe that the Guard, whilst probably not just executing
every soldier suffering from PTSD (just the ones who are so far gone that they are "misdiagnosed" with cowardice), would treat this issue with a far less open and modern mindset than current militaries.
Leech mentioned rotating units out of combat as another idea, and I think this would be a "suitably naive" way of handling it for this grimdark setting. This is, coincidentally, also how it used to be done in RL - at least according to a wikipedia article on this subject:
http://en.wikipedia.org/wiki/Posttraumatic_stress_disorder#History
In large combat theaters that see multiple regiments in operation, just switch through all of them when assigning tasks based on the "level of stress". Have them "leap-frog" between assaulting and consolidating/guarding/defending. Allow individual soldiers a day or two of rest in the encampment.
Obviously this isn't going to "fix" the soldier, but as a short term solution it might work, and that is all the Imperium is interested in.
This could well be recommendations for the regimental officers that they are not at all expected to follow (and I would expect many leaders from Feral worlds or Hive gangs would just shrug them off), but are generally respected in the more modern regiments such as, say, the Cadian Shock Troops.
