Hi Genny,
I will try to answer your questions in short.
Q: Is there a way to populate existing author records into Person entities?
A: Yes. Create a collection for it. Then import all your Person data into that collection. Don't forget to add metadata dspace.entity.type:Person to each record.
Q: collapse/combine before implementing?
A: surely, it is better to do the cleaning before transformation.
Q: can they be deleted/combined after implementation?
A: yes, but better before (you won't do a mistake when creating a relationship)
Q: Other metadata clean up recommendations before proceeding?
A: Nothing special. Just clean and tidy your metadata. Don't forget to add dspace.entity.type:Publication to Research Output to trasform them from Items to Publications.
Q: Once implemented, would we need to create a Person entity record for each new author prior to creating a new publication record?
A: Yes. (see answer below)
Q: Are Person entities required for all authors, even external collaborators?
A: It depends on your strategy. It is possible. But as best practice from our implementations, we only create peson entities for institution authors. We leave external authors just as plain text.
Q: Recommendations for handling Person entities for self-submitted records that dspace administrators may never see?
A: I see person entities as catalogue records. So I would not let Person entities for self-submissions. A workflow can be defined for administrators to be aware of as well.
I hope my answers help.
BR
Fatih