I suspect that the problem with the drains is that they are not at the same angle as the slope, plus they have a vertical upstand.
A concrete ramp starting off relatively flat and then running into the ground at about 70 degrees to the horizontal, with a sort-of sawtooth effect at the edge would probably work better than the current arrangement. Certainly worth a go, anyway.
Some new grilles wouldn't go amiss, either - probably welded into place.
Mind, I'd guess the traffic levels due to genuine FoC work are pretty minimal in the grand scheme of things: I'd also suggest that one-off casual visitors are more likely to cause issues than regulars, as seems to be the case over the other side of the valley.
I suppose the other answer might be to negotiate a 'toll' access with the fee going to the upkeep of the road? That said, I doubt anyone would actually contribute, as is oft the case with the donations - "would you chappies who are propping the roof up please mind getting out of the way so we can have a look around?"... ::)
Hello again darkness, my old friend...