Since {S2} new owner is willing to step in to help out financially, it would make sense that {S2} get a portion of the depreciation expense.
That's not an option. You, and you alone deducted all the prior years of depreciation. Therefore when you sell or otherwise dispose of the property in the future, it's you and you alone are required to recapture all that depreciation and pay taxes on that recaptured depreciation.
Remember, depreciation is not a permanent deduction that you get to "deduct and forget". It has to be accounted for at some point in the future, by the individual or entity that took that depreciation deduction.
However, I seriously doubt the TurboTax program can "correctly" handle the situation mathematically. That's why I believe you will need to use TurboTax Business to complete and file a 1065 partnership return if you wish to continue to use TurboTax to track and report this stuff.
Even then, because of the complexities of your specific situation, I'm not even confident that TurboTax Business can handle it correctly. Therefore I highly recommend professional help for at least the 2020 tax year in dealing with this.
Even the tiniest of mistakes will grow exponentially over time. Then when the mistake is realized, usually in the year of sale or other reportable transaction, the cost of fixing it will be high, and will make the cost of having obtained professional help at the start seem like a pittance in comparison to the potential back taxes, fines, penalties, interest and any other items that may be assessed for your blunder.
