I would trim around the hole, leaving a 1/2" next to the seam lines along those 4 pieces, removing the quilting as needed. Make a new block of the 4 half square triangle units, turn under the seam allowance around the edges, replace the batting piece and stitch the new block down by hand. Then I would resew the quilting :D:D:D
I think it will be an easier job than you think, and only a magnifying glass will show where the patch is...at an arms length, no one but you will see it :wink:
You can Elmers glue baste around the edge of the patch to help hold it in place while you stitch it down.
I would also consider basting the batting patch to the rest of the batting too, to make sure it doesn't shift.