I think you may have just run into some bad luck there OP. It’s very possible everything the dealer shop told you happened, did happen. And while it may be possible to repair the cat problem with the threads bunged up like that, doing so may be illegal, or illegal if done at that location; i.e. it may have to be done by a state certified exhaust system repair shop, b/c the repair is emissions related as posted above. So the cost to repair may end up being more than just buying a new cat.
hmmm … if all that turns out to be the case, and checking with another shop for a second opinion is a good idea, probably best just to have it all done, and pay up. Grin and bear it in other words. You’ll end up w/a new cat and a new O2 sensor and should be good to go. If your Focus is more than 4 years old, suggest to find a well recommended inde shop to service it going forward. Dealerships tend to be geared up and specialize in servicing new cars under warranty. Older cars not under full warranty, that’s often not their priority.
The shop who installed it may have had a problem with an oil filter coming loose before, and grumpy customer asking for a new engine. So the shop owner would naturally at that point order their techs that whatever else may happen, oil filters must be installed so tight they will never come loose … lol …