Minecraft – Does Minecraft need to unpack its launcher on every launch

You actually can use a batch file to change %AppData%. In a batch file, changing %AppData% is not global; it only applies to the commands in that batch file. (It would be a massive security hole otherwise!) So if you want to go that route and use a batch file, you seem to already know how to do that and you don't have to let worries about global environment changes stop you. For multiple instances simultaneously, use multiple batch files with different %AppData% settings.

I use this method to keep my and my kid's Minecraft installs completely separate, so that our profiles, worlds, and even jar versions are inaccessible from the other's launcher.

Alternatively, if using the new launcher is an option, this is easily done within the profile settings. I detail the method in this answer on a question about keeping different game versions' worlds separate. That's exactly the method I use myself for keeping multiple different Forge-based modpacks separate.

The two methods can be combined for even finer control over world separation, if that is relevant to your use-case.

Attempt to launch it as with administrator privileges. right click > run as administrator

If this doesn't work then delete your .minecraft folder. To find this press the windows + r button at the same time. This will bring up the run menu. In here type ℅appdata℅ and run it. The .minecraft folder should be at the top. Open it up and delete its contents. This will destroy your saved data ( although you can back that up). After this has been deleted reinstall minecraft and restart your computer.

To back up your saves for example simply just copy the world folder to your desktop and then add it back into your new installation when you want to access them again. DO IT NOW BISH