Time for a helpful tip. Typically when picking up stitches around the thumb, whether they're for the thumb or the hand, you end up with a little hole between the picked up stitches and the original live stitches. This can be remedied somewhat by picking up an extra stitch in that space and decreasing it away on the next round, but sometimes that isn't enough and there's still a gaping hole.
The good news is that you have a built-in darning thread - the tail from where you joined the yarn. Before you go to weave in that end, use it to sew up the hole, and the trouble is gone.
In other news, the top decreases worked out perfectly.