Never heard of sinus motorkote as a rust inhibitor/protective material for metal.
Plenty out there on using Amsoil HDMP. Especially in the Ford Raptor world IIRC.
Fluid film will be affected by water wash, so undercarriage and wheewells/fenders is not a great place. Other items prone to rust it can do great on.
Example - some bolt heads seen in the door jamb of my 2015 Accord were showing rust. Application of FF has done great there.
Not sure what your intended outcome is. You mention beach - so driving on the beach? Otherwise not sure you really need long term protection from salt air.
The issue with Amsoil HDMP is that it is a wax, can dry, and can let moisture diffuse under it. Oils and lanolin greases are better in that regard. But to protect exposed metal surfaces from other media blast type damage, HDMP can be good and only pick up marginal dust. CRC marine corrosion protection and LPS 3 can also be considered in these applications.
Inside of doors, welds, hood/under hood, etc, oil sprays and FF can be better. They will creep more which is more of what you want, IMO. If you want to propert wax spray the inside of panels, you really need to remove the interior.