1. Version 9.0 is 64-bit only, so it's just the addins64 directory that is relevant (the addin directory is never used).
2. The file scan is for *.dll, so renaming the file means it won't be found.
3. It appears that Windows 10 allows a file loaded through LoadLibrary() to be renamed. Maybe it has everything loaded in memory. Anyway, nothing I can change.
4. There is special support in the IDE for debugging an add-in, using a second copy of the IDE where only the debuggee DLL is loaded. This is the recommened practice, not (mis-)using the production directory addins64. Only copy to addins64 once the add-in is properly debugged.
5. Yes, as of 9.0 add-ins are activated by default. This is a huge improvement IMO.
Bottom line: I can't reproduce, and I see no obvious problems here...