The acorn nuts were probably over-torqued IMHO.
Given the way the pumpkin mounts to the SA I would have thought a repair could have been effected here. You would need a good quality stud extractor kit & matching drill to remove the broken studs from the FD.
The SA could be sleeved out if necessary, but I suspect it would be OK without.
We will never know, not being able to see the damage first hand.
As a retired machinist, part of doing a job like that is being able to jig the diff so you can drill a strait hole into the remnants of the studs. And how much material was damaged on the diff at the threads for those studs. I've done plenty of extraction jobs, and used an EDM to burn out the broken bits, even carbide. But it can be a very time consuming task.
Sleeving the SA is easy, but again, jig to hold it correctly so you can drill/bore out the holes to prepare for pressing in the sleeves. Ideally you want to bore the holes oversize w/in .001" of the sleeves so the sleeves press in nice and tight. That also means keeping the sleeves to the same size too.
With the right equipment sitting around, it's still a time consuming job and if you can't do it yourself, but are paying someone else's billable hours to do, it's cost prohibitive. Then examine the liability aspect of it. I doubt the dealer had the desire or ability to really do a first rate job and accept liability for it.
Sure, free hand drill the studs and hope you are able to extract the broken bits past the damaged threads w/o snapping the stud extractor tool. Then, if you managed to get that lucky, clean up the holes, if there is enough undamaged threads. Then install new studs with a thread bond/filler like 263 Loctite or perhaps 609.
Even if the thread damage is excessive, it's still possible to TiG weld them and re-drill/tap the holes. Of course, you really should strip the diff down to the bare housing to do this. (and it has to be CLEAN) And again, it's really going to need a proper jig to do this right and hold the diff perfectly solid and square to drill and tap.
And you can get away w/o sleeving the SA, but at the end of the day, doing a DIY repair is very likely going to produce an end result that you may have to keep an eye on and hope doesn't fail. It may never fail. Or, it may break again because the studs were not well supported. Sloppy holes in the SA might contribute to other issues over the miles. It becomes a "pay your quarter, take your chances" type of job.
With a good, well equipped shop, all this is pretty strait forward. The dealership is not that place. And the cost to have a machine shop do this work would be significant. At the end of the day, it's still a repaired set up. Liability again. If it fails, the customer is going to go back to the dealer, not the machine shop.
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Good job on finding some used parts for a reasonable price. I hope the swap goes smoothly. Gives you a chance to see what the SA bearings look like and do a proper grease job before installing the new parts. Read up on that task before you dive in, there are some aspects of the bearings that can catch you if you're not aware of them.