Mass-effect – Are there any powers that don’t actually work on Multiplayer? Or is it a glitch

Firstly for an initial point of no return - as soon as you get access to the Sirta vendor in the Hospital ward buy a/the present/s, completing missions removes them quickly before you know their value.

Secondly before beginning Priority: Tuchanka you should finish off as many sidequests as you can, since completing it renders some sidequests as failed. Don't panic about having some remaining in your journal however, a few don't have their locations unlocked for retrieving quest items until later.

Thirdly Priority: Cerberus Headquarters is the point of no return for the rest of the sidequests. You can still use the onboard vendors on the normandy to purchase goods, but because of the price markup on the ship it's better to visit them directly for whatever you need before commencing this mission as well.

This is technically possible, but for some titles it requires software designed to circumvent basic built-in security measures in the PS3. Not that there's a PS3 police knocking on your door, but if you want to be really technical, using another player's save on the PS3 network is considered a violation of their TOS.