The forest cover quest is the worst of them all, in my view. It just gives you a large area, without any context (terrain), and you can easily pass by tracks at a fairly short distance without being able to see them (especially with spruce hiding most of everything), and the tracks can only be seen in reasonable light conditions, which aren't available for much of the day for large parts of the year. The robber quest doesn't specify any terrain either, but in that case you have both tracks and the robbers themselves (which are always more than one), which reduces the risk of missing them when visiting their tile quite a bit.
Explicitly telling the NPC that you give up speeds up getting a new quest, as the game doesn't have to time out the quest before generating a new one (or maybe even start the cool down counter).
The only quests I've had in parallel with other quests are the ones with a very long expiration time where it can take months to actually be able to perform the quest (e.g. acquire wealth to be able to afford a silver item and then actually find someone [typically a trader] to sell you one, before finally being able to perform the quest itself).