Runways are bright yellow in the app, and if the comments in the skin file are to be believed, they are intended to be alternating-yellow-and-black lines. (There's a similar comment about cross-hatching railroads.) So I wouldn't worry about that.
Taxiways do reduce the distinctiveness of runways, so I agree with their exclusion. I just think it would be nice to have both, with runways painted with thicker lines. Adding taxiways increases the distinctiveness of airports. But that's just talk—I certainly don't expect an additional type just for that.
Internal roads in airports should never be mapped with any drivable type whether they're taxiways or not. That's been an overarching theme through this thread. Although admittedly I'm not sure what you mean by a "dual use taxiway"; if it really is a taxiway also used as a public road, or as a small airport's parking lot, then that's fine.
Generally I would just say "don't map taxiways at all". In your linked example, though, I would agree that it should be mapped with the runway/taxiway type, since a car might face an oncoming plane there.
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