I offer no opinion on welding vs not welding.

But I believe an overly tight chain could have caused this. The chainring and cog are not necessarily perfectly round, and so if you tightened the wheel at a point in the chainring/cog rotation where the eccentricity puts the chain at lowest tension, rotating the chainring and cog could pass through points where the chain is at extremely high tension, which would put a high compressive force into the chainstay, esp. the right chainstay. Aluminum does have a high compressive strength, but the chain cycling through low-to-very-high tension repeatedly while riding could push the chainstay towards a fatigue failure. If there is some defect or notch or scratch or heat affected zone (from welding), failure is all the more likely.