After reading your tale, it seems like the sales dept. did one heck of a sales job on you, convincing you that the "new" one you would buy and add attachments to would NOT be as bad as the floor model you were using at their place! I think I would have walked away at that point.
Having said all that, which is certainly no help to your situation, let me say that we ran into an almost identical situation with leasing a piece of equipment. It wasn't a bad piece, but circumstances ended up where we couldn't keep the machine and NO ONE wanted it back! We ended up putting it in storage, and the lease went to "Collections" after many back and forths of why I did not feel I had not gotten what I was paying for and refusing to pay any more. Just to let you know, that DID NOT end well for me, and we ended up paying about 3x what the original lease was.
Secondly, if the company is willing to take it back for 30K and you could only get 20K selling it yourself, I think I would send it back to them, take the 30K "credit" toward the lease, and cut your losses. I KNOW it's a lot of "losses," but as I stated above, it could end up way worse.
Sorry to hear your sad tale. I'm very, very shocked to hear such a thing about Roland. I honestly have never heard them being this obstinate about a product. Disclosure, we have a Roland instrument, and it is a workhorse.