Vimm's Lair probably has whatever PS3 games you want- google should find that site easily. I have no clue on DLCs- I don't think I've ever emulated a PS3 game that had DLCs so I can't really help.
RPCS3 is very user friendly as far as emulators go so you might be surprised with what non-techie people can do with it. AC4 multiplayer is listed as compatible with RPCN so you could probably play it multiplayer, it's just a question of actually finding people playing it.
As for Xbox 360 emulation: again, you can get files from Vimm's but I'd suggest you let that cook. Xenia is the only 360 emulator I'm aware of. It definitely works, but it's still in rough shape right now. Needs more time. I know OG Xbox emulators exist, but I've never looked into them personally.