However you explain it, it’s commingling of funds, and the trust fund is in name only. It’s still a piggy bank.
The funds are only "co-mingled" if you consider all taxes to be deposited into the same account. They aren't.
The equivalent is revenue paid into a business (yours or someone else) was borrowed by you personally, and transferred into your personal checking account. As long as the business tracks that loan as an asset, and you track the balance as a liability, there's no co-mingling of funds.
As you pay off the loan, the business gets the cash, and they reduce the value of the asset. This is basic accounting. It's not rocket science.
The problem with Social Security is the promise (i.e. legislated benefits) is larger than it can deliver. In accounting terms, it's actuarily unsound.