Yea. Sorry Brasso, Just finished my 20 day shift and am drinking, so my reading ability is impaired
Short answer: Yes they retain their powers, They use the Spirits stats for mental attributes. HOWEVER they
may have some memories of their host floating around in the their soup of a brain, which means they may remember people, places, dates, facts and other things... which can get really creepy when that true form spirit that took over your best childhood buddy starts reminding you of all the good times you had together.
(yet another reason to burn the things to ash!)
As to the augmentation of physical stats, that is hotly debated and I don't know if there is an errata about it. The 2 lines of thought are:
The limit of +4 (max) augmentation still applies.
The limit of +4 (max) does NOT apply as it states that the force is added, and the Force could be higher then 4.
I would only say "Be very careful!". Inhabitation is usually just an Insect spirit power so it is not something your players should be doing, and as a GM if you add the full force to the stats - which may be technically valid- could make the critter unreasonably powerful for your team... I advise caution. (But then again, I don't know your players and they may be able to handle it as well)