Honestly, let a reputable, well known smith look at it. The 2 original rear sight holes are already proven to be safe. It's the other 4 practically in line that bugs me. Where there were 2, there are now 6, all lined up in a high pressure area. Depth of holes and shape at the bottom of holes is a concern (stress risers).
If you choose to shoot it after the smith says it's your choice, don't cover that area up. Keep your eye on it for any crack or bulge if it appears. Frankly, gas pressure failures will not give much warning. You would be fortunate to catch it before it bursts, if it ever does.
Plugging the holes is fine using typical plug screws. Welding will not work. Do not go there.
Sorry, my background in aerospace engineering makes me pretty cautious these days. Especially if kids are around it.