1. I remember reading somewhere that if you do enough damage to the beastfolk via raids you will force them to relocate, essentially "defeating" them and ensuring you never have to worry about their huge game over inducing raid. Is this true?
2. Related to the beastfolk's deadly raid, do they just not do it when you form a tribe? Every time I've formed a tribe I have never gotten the warning event, even on playthroughs where I was total jerks to the ducks.
1. The best strategy against Ducks that worked for me repeatedly is to offer them friendship at first. Then, at a time when you feel especially strong military, launch ONE, single massive raid to claim their land (that's a special goal of raid you can choose, in case you not noticed (but I don't remember the exact wording of it)).
If it succeeds in the first try they will relocate, but there be no feud, second duck clan remain friendly and you can easily fix friendship with the just relocated too. Ducks have just one province, so it is instant all-or-nothing.
(Yes, to preserve good relations, this differs from usual tactics of grabbing neighbours land, where I would launch several "kill as many as you can" raids before trying to claim the land, and you have to take it one province at a time.)
In the off chance your well prepared raid fail you can still try again, but don't overplay this, if you make it into habit to raid ducks the beastfolk's response will be quick.
Relocated Ducks are lot less of a hassle, but you can still interact with them, I think I once even had them in my tribe after that, trough tribe member that had border with their new place (it's random). And, you certainly can still piss off the beastfolk if feud with them and raid them. But you won't have to deal with most of the most annoying random events associated with them.
2. Have no idea. I don't think there should be such a restriction, but who knows. If you can raid them for goods and/or thralls and cows two-three times a year for a decade I would consider it "yes".
Or, might be it just rise offsets for that.