re: Lawnmower motor mount has come loose from the frame. Fixable?

What did the service center say? They should have pointed you to their closest repair shop to have your deck replaced, at their expense.

Husqvarna is typically fairly good with their warranty.
A lot of times when I call and explain the situation I get A PO/RA for items that have been out of warranty for up to a year.
First bring it to a service center and see what they say.

I second (or third) calling Husqvarna and leaning on them to make it good. They've historically been a stand-up company. I've got a chain saw and backpack blower from them and they have been excellent pieces of equipment. Sounds like you got the red herring of the production run.

BTW The only small mower I'll own is a Honda. Muy excellenté!

Good luck.

This is the worst thing you can do to a stamped walk behind mower deck and it cost more than a new deck. You also lose the vacuum that the deck is designed to help lift the grass. It will not hold up due to the vibration from the engine. I seen way too many redneck fixes that also void warranties when the product could have been fixed under warranty.