1) I've seen a lot of packages of games and emulators recently for YDL on the PS3. If you install Fedora, you can always use the Livna repository for most of those as well. I haven't looked at Ubuntu on the PS3 for a while, but if you went that way, most of that stuff is available as debian packages.
So in the end, you do have choices for distros on the PS3. I suggest you try them all to see if one strikes your fancy. That's the great thing about linux - the only thing you lose by trying a variety of distros is the time you put into it, and most people have more time than money.
2) I seem to remember a distro for the PS3 with emulators already included, but I wasn't paying attention. You'll just have to look around for it.
3) No idea. I rarely use MAME, and I think the last one I used was xMAME.
4) No. There are a couple of LiveCDs for the PS3, and they only need the OTHEROS loader.
There's also a Gentoo Live CD, but it's pretty old at this point.
5) No idea.
Most of this stuff is WAY beyond this forum. You need to get this info from the forum for the distro you wind up using. For example, the Fedora Forum will have a LOT more info about running Fedora on the PS3 than this board.