For anyone tracking this issue, it's resolved. At least, there's a way to stop it happening and allow the merging of the accounts. It is an acknowledged bug with Quickbooks as to how this was allowed to happen in the first place, but the solution is actually quite simple:
1. Make the problem account (the one that has the Reserved for Default Account error when you try to merge to or from it) inactive - you will get a warning about this, and it will create a balancing transaction to "close the account"; don't worry you can delete this transaction later
2. Make the same account active again, then go into the register and delete the balancing transaction that Quickbooks created in step 1.
3. Now try merging the account with another and it should work just fine.
The issue stems from a bug in Quickbooks related to the payroll set up process. As explained to me, this account in my case, which is my one and only checking account, was marked as default for a payment account (merchant account) at some point, likely during the set up of QB Payroll, even though I do not have a merchant or payment account. It is this flag, which is NOT available as a user-controllable flag at all, that causes the merge error documented here.
If I get notification of a final fix from Quickbooks, I'll update here, but hopefully this helps anyone else who encounters the same problem.
Please note: this is not the same as problems that occur with various default settings in the user-accessible part of Quickbooks, like the Default Payroll Account. This is something "behind the scenes" that took me 12 hours across 4-5 calls to get deep enough into Engineering to identify and fix (and some persistence too, so don't be brushed off by general answers). Also be aware that - as well-meaning as they are - several of the posters on this chain offered simpler solutions that actually made it worse and are not addressable by the user, so while I thank the community for the well-intentioned support, if you do encounter this issue, use the solution above.
