David, thank you. I know what you mean.
No, FCP 4.5 can be installed just fine...But it launches the app only after Esc(aping) a window that says some nonsense, i.e. you have to press Esc to the first window that pops up after the command to launch it, and then you're all good.
As I said before, the only problems I've been encountering so far are related to not beling able to launch LiveType, DVD pro, and Motion. I'm running out of options and after a scheduled visit with an Apple rep today I'm just going to forget about using LiveType on my MacBook Pro and do it on my old MAC (a different computer that's compatible with this version of FCP).