Let's not over complicate this. The <40' powerboat Market has been trending, and will continue to trend, to outboard power. The limited >40' market will be twin pod or single screw diesel. Brunswick knows this very well.
Sea Ray has had offerings, but isn't really a player in any of the 3...
Helm seat - There is a bushing. Assuming its still there a little play is normal.
Halon - True Halon is no longer commercially available. You can find a company to use reclaimed Halon, but I would recommend just making sure you have newer fire extinguishers handy. I have one mounted under...
When were the risers and manifolds last replaced? A sticking exhaust valve in an SBC, either the number 7 or 8 cylinder, will cause that symptom you're describing. The sticking valve is usually a result of small amounts of water leaking back due to a riser on the way out. MMO may help...
You really can't lube those cables, however you may want to consider replacing engine to drive shift cables first, those usually go before the helm cables go
Agree it's more likely to be the limit switch. You can override the switch with the trailer up toggle, which it sounds like you're doing already. You can simply test the senders with an ohm meter and a helper. Pick up the Clymer Mercruiser service manual it simply outlines the procedure.
I have a 2004 with the same propulsion package. After having the merc dealer replace gimbals and align drives, then have the props tuned, still have that same minor resonance. Used to it now so don't really notice it