So after 3 and a half weeks of diagnostics, replacing/rewiring control units, etc. they finally agreed with what I told them on day 1, the SLS needs a new transmission. I knew this day one because at least 5 SLS owners that frequent SLS message boards (something probably less than 1% of the 2400 SLS owners in the US do) had the exact same sequence of events/symptoms as my car and every one needed a new transmission. The transmission used in the SLS, 458, 488, F12, etc. is not dealer serviceable. If something goes wrong, you just replace it.
But now I have a better idea why they jump through so many hoops prior to getting a replacement: Mercedes has to commission the production of a single gear box, fabricate and assembled from scratch from the supplier, which takes somewhere around 8 weeks. Then the transmission has to be shipped from Europe to my dealership, a trip of about 2 weeks. Then it takes about a day and a half to replace the trans-axle transmission in the SLS.
So. With any luck, I will have the SLS back sometime in early February. I’ve been told the reason for this is that Mercedes doesn’t want to be sitting on a pile of transmissions that might have a design flaw they only notice down the line. Failed transmissions are sent back to be torn down and examined, which has yielded a few design updates over the years, so hopefully the 2015 built SLS transmission will be less prone to failure than the early-2012 one it came with. So far, this is the only major issue that has been reported by multiple owners. The car is otherwise bulletproof. Here’s hoping they’ve “fixed” whatever causes these clutch pack failures, because commissioning a $20-25k transmission every 30k miles isn’t something I’m looking forward to.