Thanks! While the above wasn't what helped in the end, it got me closer to what was needed to resolve this: https://ttlc.intuit.com/community/taxes/discussion/no-outstanding-mortgage-principal-on-1098-form/00...
(e.g. there was a blank field for "Outstanding mortgage principal" in box 2 of one 1098 after the mortgage was bought by a different bank - with its own 1098; yet the error "Amount borrow" missing is really bad as this amount literally isn't asked for anywhere)
