Isn't the ZorroMT64.dll already 64-bit?
(only supports MT5, I know)

Oh, I didn’t even look. In that case, that problem is solved too.

I have version S2.03 permanent. Which of the upgrade extensions would apply in such a case? The various options are a tad confusing. Or is S 2.03 ineligible for the extensions? My confusion stems from the term " active permanent license". What does "active" mean? Thanks Andrew.

MT4: I haven't tested it, but think the 64 bit plugin on the Zorro side will work with the 32 bit MT4 bridge. At least it works the other way around. Try it!
Version 2.03: All update extensions will work for that version, since the permanent licence stays permanently active.