Never fished Mendo but wanted to. I stopped by the ramp and thought if I ever backed Dow that thing I couldn't stop until I was submerged.
As steep as it looks, that ramp off Highway 20, being as wide as it is, actually works to your advantage launching that pontoon boat, as long as there is little activity on the lake near the ramp to cause choppy conditions.
And don't worry about being in a rush to launch, because some A-hole is asking you to hurry up. With your situation, they'll have to wait... plain and simple. The park ranger patrols that area quite often
Unfortunately, the launch dock, when it is in the water, is not wheelchair friendly.
Have the person launching your vessel park it sideways at the ramp, then use your own ramp from home to board that luxury liner.
The smallmouth fishery at Lake Mendocino could suit your angling skills quite well, and quite possibly a 7-10 lb. striper might decide to take you to LA and back.