Is this a major problem? I have not hit anything...

The car has been driven at least 40K miles since this damage occurred without leaks or problems. I suggest leaving the transaxle housing alone.

If the dealer were to repair this, they would replace the entire transaxle (as I believe the service philosophy is whole-unit replacement, not repair.) It is highly unlikely that this would be covered under warranty since this appears to be physical damage caused after the car left the factory.

Although your photo is quite fuzzy, it appears that the bolt is loose on the housing. Is that correct?

If you weld the crack by applying heat, wouldn't that damage the existing FIPG orange sealant? If so, you'll be worse off than now.