Draft: Fix Multistore FF check for empty test DB: with follow-up

What does this MR do and why?

An expanded version of !75616 (closed).
Potential follow-up.
More risky, as edits multiple FF-sensitive places and expands a rescue list in a Feature class.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #346836 (closed)