If you've already trimmed all of them, you can save them by cutting new squares for the corners. I'm thinking 2 3/4, but I'd do a sample with paper before I cut any more fabric. Your 2 1/2 squares can be saved for for another project - that's a common size. 2 1/2 was small for a 8 1/2 block anyhow. Okay, I just did a 2 3/4" one and you should be okay. You cut on the 2 1/2 line and you need the extra 1/4 for the seam. Do not trim off the underside until the block has all four corners done, and is pressed and squared. I find I have to sew a hair inside the line in order to have the corner be exact. You've got to allow for the seam and the little lost in pressing it back on itself.
Draw the diagonal on the square, carefully line it the square up with where the missing corner would be, put in 2 pins and sew it from the square side so you can see the line. Do not try to use that bias edge to line up your 1/4" seam.
It will be fine and your snowballs will be a little rounder, but I like that. The ones I'm working on are 2 1/2" on a 4 1/2" block. I used 3 1/3 on 7 1/2 inch blocks, so 2 1/2 is not written in stone.
Relax, take a deep breath, have a chocolate and cut new corners. Come back if you need anything else. You can't believe the questions I asked a year and half ago.
