the contributors of premature wear on a mechanical item appear to be limited to:
1) engineering platform
to islolate your issues, you may wish to review in detial all the new MB parts used in each rebuild and compare to the failures or conditions requiring the next tranny rebuild (a partial tranny rebuild or non-comprehensive tranny rebuild may have been done previously)
further you may wish to research if any conditions/old or new parts may have contributed to your B2 failure (aka caused B2 failure)
I've heard that factory MB tranny revbuilds are top quality and comprehensive- and have a guarantee- parts and labor. You may wish to spend a little more this time to go this route time and put this to rest with peace of mind.