For 1.8.9/1.12.2 install forge for the version you are going use, download & put this mod in your mods folder(note it will not show up in the mods list)

For 1.14.4-1.15 install fabric mc for the version you are going use, download & put this mod in your mods folder

Free: Go to the multiplayer screen and click the altening button, paste the account token you have in Free account token field then click the Use free token button, if it shows a username it worked

Paid: Go to the multiplayer screen and click the altening button, go to The Altenings account section copy your API Token, paste it in the API Token field then click the Check API Token button. If it works you will see one of the buttons change to a generate button and you can then generate accounts(note the API Token does not save and you need to readd it everytime you start minecraft)

Well firstly you can change accounts without switching minecraft
Secondly? If your using mac or linux you can use this mod to use The Altening

Yes, but you need to credit THEREALWWEFAN231 or WWE Client

Yes some of the fields in the code "arent named correctly" i was planning it a different way but was to lazy to change their names