Yes.
If you set these, you get both pre- and post-filter data in one log. It’s also safe to leave these set forever unless you’ve got a particularly poor flight controller.
INS_LOG_BAT_MASK,1
INS_LOG_BAT_OPT,4
Yes use the approximation if you are using throttle-based harmonic notch. It can be fine-tuned with the filter review tool if necessary.
In your graph there is quite a serious spike around 30 or 40Hz , then a bunch of noise above that. There could be issues with the frame or the way the wiring is connected to the flight controller - a whole range of things!
What size props do you have? Can you supply a .bin log?