First of all.
Raw video has only Analog ISO levels. Meaning 200, 400, 800, 1600 etc.. Setting the ISO to 640 for instance is the Analog of 800 ISO, so that is what you get. The increments between the analogs are Digital in camera gains that do not apply to the final image. HTP does not work either, it only stresses the CPU even further without any cause. Deactivate it.
ETTR if possible yes, but I would never underexpose the talents or whatever the subject is you are shooting just to retain details in the skies. Rather overexpose the sky to keep your talent properly lit.
To be sure that you are not underexposing or overexposing something, make sure to use RAW Zebras. Set the underexposure threshold to +3 EV. Anything above that should be good enough for you to grade and get a proper image out of. But you do a shoot before with something exposed just above the +3 EV mark, to be sure if that is good enough for what you want.
But in any case, I always ETTR for the sake of shadow details, if that is what you want. But if you want high contrast "Black Silhouettes" or the like, than go for full contrast, high contrast images with MLV are amazing.
Good luck.
