Update 7/6/22: I spent three hours on the phone with Maria, who was doing her best to help me, and act as a go between with next level tech support, who kept wanting me to try the same actions over and over again. But no one could actually tell me the meaning of "this is reserved for a default account" nor how or why that might be the case for a checking account, and prevents the merge per the earlier dialogue. This has now been escalated further and I am hoping for a call back today. No one can quite believe me that this is not related to payroll or online banking and that nothing in the UI explains or provides for solving this.

I'll update when I hear more.