This is all visible in the video, so I hope that Matt won't mind me answering...
1) The radio nav aids and ADF are unrelated to the PVI-800 INS nav system, so you could select whatever you wanted on the PVI-800 without losing your ADF steering on the HSI. Since the HSI actually gives you an RMI needle for NDB approaches/nav and a separate sterring caret for the PVI-800, you can reference both simultaneously.
2) The ABRIS takes certain data from certain onboard sensors, including the PVI-800. As a position keeping device, it doesn't care where the PVI-800 says you are - it uses satelites for position keeping and does not use the INS. However, ABRIS will overlay targeting data and sensor line of sight/footprint data from the Shkval, and that is what you are seeing in Matt's video.
3) The HSI provides steering cues from the PVI-800 and ADF simultaneously (see above)
4) Shkval uses the PVI-800 to know where to look...it does not take any reference from ABRIS. that's why on a longer mission it's important to be able to execute an INS update prior to entering the target area in order to zero out errors in the system.
5) I suspect that the limitation of 6 steerpoints is related to real life limitations in memory available to the PVI-800. If you think about it, it's better to use 6 steerpoints to get you to and from the FEBA, and 10 more target steerpoints to help narrow in on specific areas of interest.