i've been using spray starch or MaryEllen's best Press for years and never had a problem with attracting any kinds of bugs to my quilts. However, I do usually rinse them in cool water after quilting, if for no other reason than to rinse off markings for quilting.
As for the seam in your "off" block, Rachel, it does appear from your photo that your seam allowance is a tad too much and that is why it is not matching. sometimes seam allowances "measure" 1/4" with a ruler, but really what you want is a "scant" 1/4", or the seam allowance that takes away 1/4" from the finished piece. Thickness of fabric and thickness of the thread do affect those things.
What I find in my own piecing, as well as when I look for problems in the piecing of others who are frustrated about things "not matching up perfectly" is that often the very edges of the seams don't match exactly, so while you may have taken off the right amount on one fabric in the 2 fabrics that are being fed into your machine, the other fabric was off just a bit from being perfectly lined up and it is off just a bit in what you took away from the finished piecing. Does that make sense?
The solution to that is to starch fabrics heavily (so that they feel more like construction paper) before you cut them, and then when you take them to the machine you can almost stack them together evenly like they were two pieces of paper.
The glue basting is perfect for holding the fabrics in alignment. just a dab of glue at the beginning and end of the seam will do it. They sell glue pens which are ideal for that, as well as just a dab of water soluble glue. Elmers works..just a small dab, applied with a toothpick will work. heat set it.
Hope that helps!
Many suggestions here will help.
And BTW, the blocks are gorgeous and NO ONE WILL NOTICE, unless you are trying to win a ribbon in a show! <3 <3