Thanks for the photos of the seat binder. Almost exactly as I had imagined (and drawn above).

How is the new seat height?

Since you're considering more work on the top tube, I'd move the seat binder under the seat stays. That is a good solid place for it anyway.

It looks like that was brazed together, so you should be able to pop it apart pretty easily with a torch, and without damage. Then you can braze it back in place, and the adjustment should be good.

I like your idea of finishing the cut with a file, or even drilling it out, but I'd probably do the initial cut through the tube before brazing the bracket on, at least to get it roughly down to size.

Why did you drill a hole in the side of the seat tube? Are you thinking of mounting the bracket on the side, rather than in front or behind the post? That would be an odd solution, although it should work as long as the legs don't rub on it. The old QR stuck out a little anyway.

If you braze, you won't have to worry as much about heat distortion and penetration through, and it should be easier to work in tight spots. Oh, also, brass can contaminate your tig welds.

If you leave the seatpost in, I'd find a junk post at a local bike recycler/co-op.