There is a product made just for fixing tears. It is called Bo Nash, and I recently used it. It is a powder that you sprinkle over the backside of the tear after you have perfectly aligned all the threads involved in the tear. Then you put a patch of the identical fabric overtop of that (still on the back side) and iron it down with a pressing sheet which comes with the Bo Nash (and is like an applique pressing sheet.) It works very well to mend the tear in a nearly invisible way, although the mended area does get a bit stiffer than it was before. I have also heard of people doing the same thing with a patch of fusible web on the backside of the tear, but I've not done that and can't testify to how it holds up in the long haul. The Bo Nash is supposed to be a forever fix. You do have to wait a bit (2 hours, if I remember right) after the fusing before you "wear the garment" (or put the quilt on a frame, I suppose. I remember I waited to do that.)
With either of these repairs you need access to the backside of the torn fabric, so I don't know if that involves removing any quilting stitches for your tear. If so, they will have to be "re-quilted" after the repair is done.
Good luck to you! I think if you can make a nearly invisible mend, you should be able to enter the quilt in a show. It probably wouldn't even be discovered.
BTW, the L shaped snip might be from hitting the folded fabric with the rotary cutter when you are cutting other fabric and the folded fabric is laying closeby, near where you made the the end of your cut. (Does that sound like the voice of experience? I was SURE it was far enough away...
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