I definitely don't think about REM too much while trying to lucid or prolong it, since dreams outside of REM are know to exist and the total of 90 min limit makes no sense most nights. The question for me is, is the fading of a dream inevitable?

Sageous, so basically you are saying that the inability to stabilize or reenter a lucid dream boils down to the lack of experience? This certainly gives me motivation, since experience is something that can be gained, but if the cause is a biological limitation (doesn't matter how we call it, let's say a mechanism more complex than simply REM cycles) than nothing can be done about it.