Sorry, I wasn't clear on the first post. We did submit current-year estimates (Apr, Jun, & Sep) in her name & under her SS# only. So she is covered there.
The only issue is making sure the $6K refund on the 2018 return is credited to her SS#. Thanks for validating the idea to switch the names & SS#s on the 2018 return. That's what we are going to do, although there will still be some lingering uncertainty since it will be a paper-filed return.