Here are the answers to the questions which haven't been answered yet:
Marius-and-Dominik says:
To the surfaces: GP, platform, offroad and road seems ok, but road can be many things:
canyon-road, tunnel-road, platform-tunnel-road, road on the little mountain, road on ground, road in air (last two things are also very different to drive). So are all these different surfaces?
Another thing, thin road on the ground and in the air count as different surfaces or not. Because they do look different y' know.
They look different, so they are different surfaces.
Air-roads and tunnels are the same surface (same texture, same drivability), cliff- and groundroads are the same (same texture, same drivability)
Anaerob says:
What if we have a three-second platform bit, a two-second jump and then another two-second platform bit? I.e., do we count time driven on the surface, or time between surface changes
That's legal because air is no surface, i.e. air counts as 0 seconds of the previous surface.
Mandark says:
I have a 2 seconds off road, drop and 2 seconds off road.. is that legal or not ?
Legal.
Mandark says:
Are these different surfaces ?
They are different textures, but they aren't affecting the driving in any way...
Different textures = different surface, as simple as that.
Enai says:
The top of the narrow concrete blocks (5-1-X) actually has a different texture than the big concrete blocks (3-1-X) even though it looks almost identical. Is almost good enough or are these considered the same?
They are considered the same.
Did we overlook any question?