Direct Head Tracking shouldn’t be visible in Skyrim, please ignore for now if it doesn’t work.
Multi-threaded input didn’t do anything useful anymore since async timewarp was introduced, leaving the option visible was an oversight. For async timewarp it’s important to sample the head tracker at specific points in time. I’m surprised you found, with asnyc timewarp it usually caused judder.
Fluid Sync is mostly important on Vive. On Rift it can give a slightly better experience async timewarp experience, but in general just leave it off on Rift.
I would generally advise not to tweak too much although that might be tempting. The default options especially in regard to input and direct mode are the right ones for your headset.