a) I do not know how to change encoding on Notepad's text files...

Can't help you there. You should be using software designed for managing CSV files. Google Sheets is free and so is LibreOffice.

b) If I try to export Scandinavian Å Ä Ö they turn up as Ã… Ä Ö (which I can readily replace back to Scandinavian characters).
But will it work the other way round (I.e. I upload Ã…, and it turns up as Å in the name field)

That's probably because the software you're using to open the CSV file.

a) The Gids field is exported in the following format: "[""2"",""12"",""13""]"
I take it that I can upload it in this format instead: "[2,12,13]"

Same issue as B. The software you're using for editing a CSV file is not very good. Notepad is great for a lot of things, but editing CSV files is not one of those things.

b) And Empty fields are exported as: ""
I take it that I can just make one delimiter follow upon the next to upload an empty field value, as so: ,,

Empty string is supposed to be empty string as it is exported. Do not interpret this as a chance to format it however you want. I've given you the CSV configuration above.

c) The Params fields are exported as: "{""admin_style"":"""",""admin_language"":""""}"
Can I upload the same data in the form: "{admin_style:,admin_language:}"

Use the format it gives you on export. No you cannot just make up new formats. Use software designed for editing CSV files and on open provide it the configuration given earlier. LibreOffice for example will ask for all those parameters on opening of the CSV file.

Are you really certain that your answer referring to the parser is correct?

I wrote the entirety of the import/export code, so yes.

(I have not yet come across a string parser that would give a different output when parsing "[""2"",""12"",""13""]" v.s. "[2,12,13]" ,
in both cases the field value would be [2,12,13] , wouldn't it?

It won't display like that in an actual CSV editor where you can specify the string delimiter as I've already explained the settings required below.

www.joomlapolis.com/forum/147-potential-bug/244488-odd-error-message-while-try-to-import-users?start=0#327161

Stop using Notepad and use software actually designed for CSV files. LibreOffice is 100% free and works on Linux, MacOS, and Windows. Also works fine with Google Sheets which automatically detects the string delimiter as ". If you insist on using Notepad then stop using CSV export and use JSON or XML.