I think that depends on your non-Latest 5*. Like for me, if I were to crack my hoard now, I’d likely set Thanos as my BH. That’s because he’s at lv450, so I would want that extra LT. If you have a Classic at 11 or 12 covers, you might want to set that character as your BH as well. If none of those apply, set the 3 Latest as your BH.
As of now, they’ll only swap Latest 5*.
Likely yes.
306 is a pretty safe buffer to crack open a hoard, since on average you’d need 260 to hit 13 covers on all three Latests, assuming no dupes or BH. With 306 pulls, on average you’ll get 15 covers of each 5*.
For 2 - they will only swap a cover for another one of the same character. If you have a 14th cover of a character you can in theory use it, so they won’t swap it for another character or anything like that.