Aaaargh!
#15
#17
There's an easy way out of this one. I would take my "duck bill" scissors and clip very close along the stitched line. No need to rip out the stitches. Trimming along the stitches will remove one side of the piece and the other side may just rip out or another trimming along the opposing side may be necessary to set it free. A tweezer can remove any remaining threads.
#19
I have sewn the corner of the backing as I quilt. I just take small scissors and cut around the stitches, take a needle and scrape at the backing corner to ravel it and take all the extra fabric out from under the quilting stitches. I use to do it alot, now I try to pay more attention.