Correcting some previous work that was done.

Question in regards to installing metal bullnose (with paper front).

1) I understand that I should use the all purpose USG green box for this. I have USG Blue box, and I have 20 minute hot mud as well.

2) It seems that simply with enough mud on the corners (with perhaps some "buttering" on the bead) that it will stick, but will it stick enough to squeeze tight? Just want to make sure that a good amount of mud is all it should take to hold tight? I'm trying to avoid sitting there holding it against the corner to assure a tight fit. May be overthinking it, but let me know if there's nothing else involved.

3) I have a 120-140 degree rounded/bullnose corner. I noticed that the big box stores do not carry a bullnose for this angle. Should any dry-wall specific store carry it?