Time to find a better dealer who can look at a parts book? We have a
B7500 HSD (4wd Hydrostatic) and it most certainly has a clutch. Here is a screenshot of the revalent section of the parts book from the Messicks parts app:
View attachment 574731
Sure looks like a clutch to me...
If you put it into low range does it push any better? Does the hydro stat whine when it is stalled out trying to push something heavy?
Dealer should not need to open up the transmission to do a couple of basic tests on it. There are external test ports where you can test the pressure of the various hydraulic pumps inside the transmission and make sure that they are pumping hydraulic fluid like they are supposed to.
If your mechanically inclined, I believe that the shop manual can be found on Kubotabooks.com that will tell you where to find the transmission test ports and you could use those along with a gauge, a hose and some adapters (probably $50 total) to test the pressures yourself and to see how is working without needing to open up the transmission.
Aaron Z