Not sending SSDI payment: If I do not send my SSDI backpayment to my LTD insurance company (Unum) upon demand, do they have any recourse beyond withholding my future benefits in order to get their money? Could I get sued?

Yes, you might get sued. Under most long-term disability insurance contracts, you are required to apply for Social Security disability (even if you would not meet the requirements) and you are required to reimburse the long-term disability insurance company if you are paid Social Security benefits.

This is just another reason why group long-term disability insurance contracts are basically sham contracts. If you can afford to do so, you should buy a private long-term disability insurance policy before you are disabled.