Looks like the TIG was a bit overdone at the seat stay and started to blow through the seat tube. After seeing the new pictures, I would go with the Dremel, and a steady hand and hit each little bump. Even of you overdo it a bit on those bumps and take of a tiny bit extra, it should be fine because there is seatstay and weld backing it up.
My Dremel came with a mini-sanding drum which might work for this.
So, carefully grind them down, then finish with a dowel and fine grit paper.

Only problem with a Dremel is the bit might not be long enough to reach the deeper bumps.

Additionally, while sanding or Dremeling, perhaps stop periodically, and mark the inside of the tube all around the area with a "Sharpie". Then start sanding again, and confirm you're hitting the right area due to the Sharpie being worn away.