Hi Firstrib .. I have the same outboard as you.. and do all my own servicing on it. Im not an expert though..so take what you want from my post...and ignore the rest
There is only the nylon nut holding on my prop too..its never failed or slipped in its 10 years of life. Be careful not to over torque it or the threads may strip. Like you..I used to wonder why there was no castle nut and pin.. but I donít worry now..and its served me well.
The cover is the exhaust manifold cover..and as such..water can leak out the gasket. I would be inclined to investigate it ..if water can leak out it..can it also leak into the cylinders ? See note below the diagram from my workshop manual. Looking at your photo of the cover..I would think someone has had that cover off already ? ....three bolt heads look new..the others have paint scraped off..so they should come out fairly easy ?
The trim tab you discuss .. is above the prop.. and the reason it is there is stated in this diagram...and how to adjust it too.
Hope that helps some ..enjoy the outboard ..it should be very reliable.. I have never had any issues with mine...and it has done a mile or three ..in some rough weather too.