Is power being transmitted from the flywheel to the transmission input shaft while the clutch pedal is fully pushed in? It sure sounds like it.
Yes, there are two possible scenarios for this unwanted power transmission,
1 unwanted /un-intended contact of the clutch disc to flywheel or clutch disc to pressure plate.
2 unwanted /un-intended "drag" of the transmission pilot shaft in the pilot bushing/bearing of the flywheel
From the outside, almost impossible to determine which or both, or to correct that condition(s).
I split (AND re-assembled back to working state!) my B6200 HST myself. I work as an electronics technician, my hobby is woodworking. Far from a mechanic or heavy duty technician. Mine is HST hyrdro, so some slight differences to split, but the overall task is the same.
It was not "hard", but took some time and some concrete floor space in the shop/garage. Yes, I took off my loader. You end up with A LOT of pieces. Take photos with your phone of everything you take off. Get a bunch of big zip-lok bags, and each thing you take off put in a bag with the bolts that held it on. Then you don't get the bolts mixed up or in the wrong place. If I can do it, you probably can.
Once apart, figuring out what was happening is not easy.
The splined area of the transmission input shaft where the clutch hub rides and slides could be worn, and encounter resistance sliding along the splines to disengage. Yes, the dealer should have checked for that when you paid them a bucket o' cash to replace the clutch. Tearing into that transmission to replace a worn input shaft is not going to be cheap. I have never done something like that, I have no idea how easy or hard or how many special tools or equipment is required to properly service and assemble that transmission.
If the pilot bushing/bearing is the problem, a worn input shaft and disassembly of transmission is likely.
Your best hope is that the clutch hub / splines were inadequately lubed when the clutch was replaced, the shaft splines are not worn, and simple application of hi-tack grease fixes it. The input shaft is harder steel than the clutch hub splines, so hopefully any wear is on an easily routinely replaceable part.
With the pilot bearing/bushing, if its a bearing and the inner race is secure on the shaft, the shaft will be fine. If its a bearing and the inner race is spinning on the shaft, the shaft is probably worn more than one would like, but shouldn't be a tractor killer. I've heard of situations where pilot bearing is completely forgotten / left out on assembly, and the unit operates normally for years. If its a bushing, the shaft may be slightly worn.
That the condition existed before the clutch change, and continues to exist, I suspect inadequate or dissipated lube on the input splines.
My mid-80s vintage B6200 with a couple thousand hours would bring $5~6k in this central Alberta used market. I think its worth doing the work.