I'm sorry you're having difficulties! Unfortunately once you migrate over to a Jagex Account, you are unable to revert back to your previous login. I would like to point out tho that you can still login with linked apps (i.e. steam, google, etc.) even if you've migrated to a Jagex Account. [That's Question 2..]
So as far as Question 1, I think this may be a hardware issue, are you playing on a laptop or perhaps a very old desktop? If you cannot "end task" and it's hanging (not responding) for that long, it very well may be a processor issue or memory issue where it's trying to catch up but cannot keep up. If you'd like, please provide a little info on what device you play on and might be able to give a little more info. Hope this helps!
Once you open any of the actual RS game clients, you can immediately close Jagex Launcher (right-click Exit in taskbar) and if it's the cause of any instability for those games that should stop. Jagex Launcher only needs to be open for maybe 10-20 seconds just to launch a game.