First off, for a first project you are not doing badly at all!
But you see the advantage of a print back over a plain one, a busy print can hide a lot of flaws, however it doesn't let the quilting show off to it's full advantage but that can wait until we get to where we want to show off the quilting... still doesn't happen that often for me!
The easiest solution at this point is to stitch down the largest folds by hand and ignore the little "pookies". Use a thread that matches the background, hold down the pleat and blind stitch it down and then move onto the next one.
Depending on the front of the quilt, once you've done that you might consider going back in a square grid over your diagonal one. Or not, up to you! It was made with love and your baby is going to love it and you without perfection.
For the future, I like the spray baste, other people use glue or other things. It takes awhile to get used to it (I think most people overspray in the beginning), but it gives me the best finish when quilting it down. I layout small tops on my dining room table (when I was working I had access to a huge conference room table, could do a queen in length and just a bit of fall over the sides, some people with garages and more space like the folding ping pong tables because they fold up when not in use and you can get them for free/cheap at garage sales). If you need to use weights like canned goods to hold it smooth that helps too.
Even on a baby quilt when I spray baste I still do it in sections. I have fairly short arm length and typically only work with about 1-2 feet at a time.