There may be additional criteria that is overlooked here. I am playing a MP HTTT 4.1 game as Japan and have cored up Korea and Manchu. Wu has revolted and the Ming are -200 to both me and Wu (and pretty much everyone else). My relations with Wu are -170 as I took one of their provinces on a conquest mission CB. My ruler died and I was trying to feverishly to get some new RM's in place... to my suprise, when I checked Wu, it was very likely, and they accepted. MY new king had a DIP of 5, and I had no great men to boost this, my prestige was 40-something? MY trust with Wu was "...uphold our bargains". Perhaps the enemy of my enemy is my friend maxim applies here somehow? I was happy to get the RM as I have no intention of messing with Wu anytime soon, but for the life of me I could not understand how it was "very likely" on the RM.
I then panicked, thinking crap, they have probably have a high prestige and want the RM to claim my throne, but they have -30 and like I said, I have 40-ish. I then checked a few others that were adjacent to Ming I had poor relations with, and they too were very likely, and accepted. Assam, who has about 15 provinces was one such who agreed to the RM, and Da Viet, who has 2 provinces also married up. I checked the threat, and Wu feels threatened by me (so maybe they wanted the insurance policy of a -1 stab hot for me if I pull their card), but Assam and Da Viet do not feel threatened by me. My BB is/was in the high single digits. Maybe they were all courtiing me b/c I was the only one capable of standing up to Ming?
All in all 3 of 3 people who I had -170 or worse relations with all agreed to a RM under very likely phraseology. Any explanation would be useful!