I understand that the beneficiaries of the estate didn't want anything to do with operating commercial real estate, so it was just left to fail.

Request a beneficiary statement.  

@Wilson Pereira  Well for bank accounts you would probably just need to make the other trust the beneficiary. 

Has anyone dealt with objecting a subpeona to a land trust to disclose beneficiaries.

You need the connections or manpower to scour the counties recorders offices for seller carry backs then build a data base of those.. then you have to try to skip trace or other methods to get to the Beneficiary of those notes.

I know legal ownership and beneficiaries can increase that coverage in a number of different combinations, but are there ways you can think of to expand coverage specifically with the solo 401k's?

They require life insurance  with the bank as beneficiary .

Are there any other beneficiaries under the Trust and do they have anything to say about selling you the property and any related terms and conditions? 

I am the custodian (no 3rd party custodian) and beneficiary of both entities and can invest from either as I choose.

Best thing to do with subject to is to have the seller transfer ownership into a land trust where you are the beneficiary of the land trust.