Agreed, I have read a couple of sources where understrength units are combined into a retitled regiment.
I think there is probably little if any chance of a vetran being shipped home. Most, if not all, IG units are posted away from thier home planet due to Imperial paranoia about uprisings and revolts (and the fact troops will be more happy to slaughter foreginers rather than thier own). Spending cash, never mind the logistics, to send individuals across segments, possibly even across the galaxy in some cases is never going to happen.
I imagine that on discharge, vetrans are maybe given a small cash bounty or something like a chit redemmable for a plot of land on the nearest colony world and left to thier own devices. I expect they may get preferential advancement in the local civilian law enforcement or in the PDF as senior NCO's in the garrison quartermaster or training contingents. Some may find employment as bodyguards or in security on either side of the law, with a very few selected for work with the Inquisition.
I see no problem with officers being recruited from the most able - the Imperium is a meritocracy - the best man for the job and those with responsibility carry the threat of the ultimate penalty if it ends up fubar.
As for volunteers, there must be a few, but I imagine they may get a rough ride from thier fellow forced conscripts...