Importing evernote .enex

No. This is a limitation of Evernote's ENEX format. You can read more about that here:

and here:

Thank you, very helpful on Q2!

For questions Q1 & Q4 - what have users found is best for my use case, 8.1k notes, 159 notebooks, 12 stacks? (I would consider this a moderate number of notes but not overwhelming, but, a fairly elaborate folder hierarchy)

[A]. unique tag each stack, each notebook, then, export all 8100 notes to a single ENEX and import, search each tag, move to newly created folder hierarchy;

or

[B]. 159 discrete ENEX exports, and 159 discrete imports (one export/import per notebook), moving each import to a newly created folder in hierarchy?

And for Q3, Q5 any thoughts?

JEX is essentially a tar (or tar.gz?) containing all notes, attachments and metadata, so while there probably aren't any converters from Joplin to any other app, it would not be too hard to implement.