How's the weather in hatboro these days? I grew up near there . . . . I don't know if I can help you, either I don't know enough, or can't tell enough from what you have posted to know what might have been missed by you. But, first the iMac is apparently one of the more difficult platforms to get linux running on . . . . And, whether it is relevant or not, is the 12.10 version the "PPC" version? Other than that, if you can get to single user you might be able to find the boot parameters that will get you into a GUI . . . . I recall that when I was trying to get my iMac to run something linux I had a long list after the "live nomodeset . . . ." They were all listed when I hit the "tab" key after the first log in window, and I just strung them together.
But, the gentleman who might be able to give detailed help is "abtabt" or the OP of this thread "rsavage" if either of them stop by. In lieu of that I'd suggest the "rsavage" thread "Tester's wanted for 12.04" and try reading through there for any tips . . . which is my best suggestion . . . first try to get 12.04 running on your computer before trying to move forward . . . . rsavage was offering an .iso with the pre-compiled nv driver there, but had to take it down. But it's not the only choice; with "str8bs" help I set up an xorg.conf file that used the fb driver, and that was fine as well . . . nv is more "native" for the iMac I suppose. I now am running Xu 12.04 with an nv driver that I compiled following the ubuntu PPC wiki . . . and all is well, except for no "suspend" function, and I get a "the system has crashed" error every time I log into it . . . which I think was part of the beta version of it.
The other option is to try a Lubuntu for PPC .iso and see how that goes, "Philw" seems to have some instructions out there for 13.10?? but, since it has been so difficult and time consuming for me to get something to run at all on my iMac G4 . . . I'm sticking with 12.04 until support runs out or maybe longer . . . . At one point I was downloading the LiveDVD's of Lu for PPC and testing them out, but very difficult to get them to work; on my iBook with the radeon driver it's a lot easier to move up . . . . Good luck, hopefully someone else can jump in and offer some pointed questions; if not, you'll have to try to read more to find the right string of boot params to get you started; or figure out where you need to make some adjustment, perhaps something in your xorg file, etc . . . .