The limit is just for active subscriptions.
Outside that you can have as many mods and downloads as you like.
Unsubscribing doesn't uninstall them.
IF you are worried to refind them on the Workshop after unsubsribing for potential updates or because you lost the mods or whatever, then simply favourite them. That way you can find them as well. There is an option in "Browse", just like for the subscribed mods.

Not sure how many the launcher can display, but you can just create a storage folder for mods and move them into the "mod" folder as you need..


Automatically convert ?
No.

So while it's not required for you to do, i'll elaborate a bit more below anyway :

Not sure why you call mods not from the Workshop "local", as they all are locally stored.
Whatever. Irrelevant in the end.

All difference is them being not zipped/packed and having another line in their *.mod files.

Not sure why you mean to say that this takes a lot of time, unless that you meant to say regarding the "contain many folders" part that some are big and hence need more time to unpack. If you don't want to use Windows' own unpacker, then i recommend 7.zip,as it excepts all packing formats.

If you still want to use the zips as unpacked mods, then all you need is to unpack the zip and edit the small modname.mod file.There you have to change the second line. Change "archive" to "path" and get rid of the extension being mentioned at the end of the line.You can open them with any editor. I use Notepad+++.