I have a few questions about stealth and invisibility.

You have to be stealthed to backstab. If you are in combat, can you cast invisibility, re-enter stealth and backstab again, and then repeat? Do you need to have the spell muffle active for this to work?

It seems like you could be an unstopable killing machine that never gets hit if this works.

You have to get the Invisibility spell first. And yes, if you can't muffle your steps somehow, you won't be very sneaky.