For all you Movarth lovers, I have some bad news. It isn't so much bad news if you know... but, obviously chances are you don't go looking for a solution unless something bad happened first... Anyways, Don't-- I repeat, DO NOT kill an enemy before Movarth has seen it. This will cause him to shut right down and your only salvation is to reload. I suppose it is possible my Movarth, being a respawn, has a different shutdown trigger than the real one. It's possible, though, I doubt it.
Recap: It's fine to kill an enemy in one hit, but an instance where you go to finish it off, and Movarth is still cautioned is a big no-no.
Despite this, I am very glad his AI will remain intact through thick and thin, as long as you avoid this particular scenario. Just wonderful.
Some lighter news: I've discovered how to give thralls permanent health regeneration... I still need to troubleshoot a couple grey areas, but I'll post something when I have an accurate formula.
edit: Seems I've totally misrepresented Movarth. Just shows you need to play through a single scenario upwards of 20 times before you get a definite answer..
(Caution: Spoiler)
It's the Dwemer Laboratory, not killing enemies in a certain way that was causing Movarth to shutdown. The scripted dialogue when going about the Thieves Guild somehow interferes with his AI. I can't make sense of it. If I wait for the one guard to come through the Locked Gate, he'll be fine for the area. Unfortunately, however, beyond that he'll shutdown for whatever reason before the next two guards talking about Dwemer traps... I sufficiently wore out my patience trying to discern the cause of his shutdowns in the first area, so I'm going to leave troubleshooting the next up to someone else. My solution: Leave him out front the Understone Keep for the duration of the quest...
....It may explain my rash assumptions since Movarth had shutdown the other day. We were traveling by foot through Falkreath. I honestly don't remember where I'd come from, but after reloading today I learned he'd forgotten how to fight. Reload. Lost about two hours gameplay which isn't bad. I just wish I could pinpoint what initially caused it.