As opposed to this highly plausible violence?
https://youtu.be/wtoHjGWc2s8?t=1m36s
Star Wars has never been good at the whole "logic" thing.
Finn's still a stormtrooper. We never really see anything lower, but they're nominally elites. Roughly analogous to a Marine. Every Marine regardless of their MOS is an infantryman first. Even the Marine who spends most of their day cleaning (which is a lot of them, I assure you) is trained to kill. Finn may work sanitation, but he's still a trained combat unit. And Kylo had just gotten shot with a weapon that can down a TIE in a single shot. Dude was kind of distracted by the whole "dying" thing. Finn standing up for forty seconds, most of them on the ropes? Yeah, that's fine.
Apply the kind of reasoning you're using to the OT, and it will not hold up. At all. Han should have died in the exogorth because he walked out into a vacuum with nothing but a breathing mask.
And yes. Yes, you can absolutely be a Sith Lord and an apprentice at the same time. Darth Vader was a Sith Lord. The novel Lords of the Sith is quite clearly plural, and about Sidious and Vader.