In my opinion, I wouldn't really want to tie legitimacy to monarch points. Yes, there is a natural trade off to that system - those are points you could use for expansion or ideas that you will never get back - but I'd also like to discourage the kind of gameplay that involves pushing a button to solve your problems. It works okay for stability, but less so for war exhaustion, which is too easy to pay down with diplo points. I would prefer something like pay X amount of points to counteract X amount of war exhaustion, which is how war taxes work. Inflation should work like that too.
By the way, it is possible to get out of a civil war quickly if you have a lot of money. I got into a civil war once as Orissa (one of those nations that, ironically, has a national idea for legitimacy) after decades of bad luck with heirs. Civil war progress does carry over to multiple monarchs, by the way. So you can spend years climbing out of that hole, and then you're right back down to 20 legitimacy if your king dies without an heir. If the only progress toward a civil war comes from having over 10% OE, you can still expand through vassals, but I wasn't willing to do that for years, potentially decades, so I just said fuck it and accepted the disaster. The civil war itself mostly involves trading money and prestige for legitimacy, so if you have lots of ducats lying around, you can get out of the civil war relatively quickly and with most of your manpower intact. It cost me about 1000 ducats to go from 20 to 90 legitimacy. It's better if you have that money stockpiled, but a few loans wouldn't be disastrous. The +10 unrest is nasty, but if lots of revolts do occur, then I would consider declaring war on a neighbor and letting them choke on the rebels.
