Can I go back to 3.41 after having 3.55? Yes, but it involves formatting your PS3 HDD before downgrading.
Are all working games working online?. Most games from what I've heard/read are working like their supposed to, but I'm sure you'll always run into the one that don't.
For gaming online, It's probably best to use the original disc and if it's worth playing online then it's def worth the support and your dollars to ensure online play!