Exactly, but it isn't too unlikely that you can reach them by land because someone involved will probably get military access from Jianzhou.1. And how exactly would you reach them? A naval invasion with your 10 cogs? There's also The point of historical accuracy. WHY does this truce even exist? There is zero historical reason for this to exist, it's merely a bandaid covering the real issue of bad AI not handling hordes properly.
There actually is a historical reason for the truce to exist. Joseon attacked the Jurchen in 1433 and conquered northern Hamgil.
Yeah they are. The but the dev is probably low because they wanted to implement as war ridden land.3. Korchin and Mongolia are in the Tartary region IIRC. However you are correct about Yeren and Nivkh having 3 dev provinces, I forgot that Manchuria got moved to East Asia last patch. Still makes no sense why Korea has 3 dev provinces however, Northern Korea is nowhere near as sparsely developed as Siberia.
This isn't too much of an issue imo. I play a lot in Japan so I know who to definitely not improve relations with. I agree that it is to some extend a waiting game, sometimes more, sometimes less. But what else are you gonna do anyway :b4. You're still stuck with them until they get into a war that you can enforce on, which means they need to be the war leading offender, which is rare considering their tiny size. Might as well wait until Japan unifies before you get the chance to do so.
AI Japan takes centuries to form, not really keen on waiting that long. It's also an easy way to get the foot in the door.