@jcd4 

RE: Is there a simple way to convert QBO files to IIF format -- which as you state will be allowed for import still after May 2024 on QB Desktop 2021

There's no built in way to convert a file, but a good way to convert QBO files to IIF files is to use our BRC OFX/QFX/QBO to IIF Converter 

RE: ...and if so, once that conversion is done, and I import, will QB Desktop 2021 recognize the transactions from that IIF as coming from the same source (bank or credit card) as the QBO it was converted from. Will it apply the transactions to the correct account that bank or credit card was attached to when it was being imported through a QBO format.

Yes, the account (bank or credit card) is added to the IIF file and then the transactions import directly into that account's register.