Do vehicles load/unload faster the more that the resource is covered?

No, as near as I can tell resource coverage does not affect load/unload speed. Resources at a station naturally degenerate over time, so to get a higher% of hauled production, you need to have one vehicle loading at the station all the time. For part 2 of your question in one of my games, I had a station cover two coal mines with just 1 square on each, and I still transported 91% of the production.

However, one thing that does affect load/unload speed seems to be the station itself. If your train, for example, is longer than the track it seems like it takes longer to load/unload.

Also, all the cars on the train load simultaneously, so 10xoil tanker train will load more oil per 'tick' than a 5xoil train, but it takes approximately the same amount of time for both trains to reach 100% capacity (assuming enough oil at the station and it is long enough to accommodate both trains).