Yikes! That doesn't sound good. Cutting out the crack and inserting a new piece sounds like it would weaken it, but Im not an engineer, so I hope they know what there doing.
Rob
welds are usually stronger than the original material
if anything on the part breaks it probably will not be at the weld.
Welds are not stronger and will crack almost 100% of the time. This is why little to no welding is done on aircraft structure. Every aircraft has damage to some degree. And I would be suprised if problems like this didn't happen.
Cutting out and replacing the piece will only make it stronger. No engineer is going to repair something by making it weaker.
And yes they will know what they are doing, or they would be sitting behind a computer.