You seem to be basicaly correct. After drop, if you do not import, the next drop will add to or replace the prior tempoary json tiddler containing all the payload tiddlers.
Import and cancel delete this tempoary payload tiddler. Which may supprise you is $:/Import
I belive the tempoary payload is not saved back to disk when you hit save.
You can change the way the way the $:/Import tiddler is built on drop and can add your own import tools.
Regards
Tony
I do not expect it to have much impact because when you import variables they are imported for that tiddler, but when you creat global macros every tiddler "imports" the global macros at render time. I use hundreds of global macros with no problems.
The only caviet is you could make a macro that had an encyclopedia in it, then you may have a problem.
Regards
Tony