Originally Posted by CrimsonEclipse

What's the purpose of the repair?
To prevent frame failure or future cracking?
To prevent corrosion?
Improve looks?

If you want to keep the water bottle cages, your idea on rivnuts is really the best option.

If the any of the holes is a little off, you can slightly larger hole to properly center it and install a slightly oversized rivnut
(last resort)

Welding just seems unnecessary
(especially if aluminum)