Now that you have lots of suggestions - let me first say - been there, done that!! Welcome to the club!!! And I learned to make a sample block before cutting all my fabric ~ as you just did!......
I'd make the quilt center smaller by trimming off 1/4" on all sides of the snowball block. That'll make everything work again. That is what you did by cutting the triangles in half to begin w/. Do this w/ only one block to make sure you have the right stuff. While time consuming - I'd do one piece at a time to maintain my accuracy.
Definitely starch when pressing to prevent stretching of the biases....I also throw a pin in there to prevent me from stretching even further.
If you need to make the quilt bigger, add an additional border in there.....
I cut as I go now because of getting burned a couple times....You'll be fine - breathe!!