Agreeing in turn with Padraig, for the most part - but with regard to Tolkien's support of Franco, I think a crucial part of this is that some of the more hardcore leftist/anarchist brigades in Spain at the time were anti-religious to the point of going around gunning down nuns; to a devout Catholic, any regime that opposes this can only be preferrable to the alternative. Though in fairness it's conceivable he'd have found Franco's regime acceptable to his sensibilities even absent the threat of militant atheists, I don't know. It goes without saying Franco's regime, as repugnant as it may have been, was nonetheless pretty far removed from Nazism.
On your point about 'good' monarchy replacing the overthrown 'evil' tyranny in mainstream fantasy, surely an exception to this is the Star Wars series - I say this on the basis that these films aren't really science fiction at all, but fantasy (fairy tales, really) dressed up as sci-fi because they have laser guns instead of long bows, Death Stars instead of magic superweapons and so on. I have no idea of George Lucas's personal politics but it's notable that he sets up a by-definition-good Republic against a by-definition-bad Empire. Or should this be viewed through specifically American eyes, in the sense of the fledgeling 13 States fighting for their freedom from tyrannical old Mother England?
Also, 'galactic feudalism' may be a well-trodden path in sci-fi, but it also features in Dune, which is an immense book.