Hello there! I know this is somewhat late (3 months), but I think that some response is better than none. I have read through (but not played with) the rules you mentioned, and they really do nothing to explain what a raid is (although they do cover how to order a raid and what happens when you successfully complete one).
1) For your first question of which Scenario to use, you can use whatever you want. I am assuming you are the CM and thus can make up whatever you would like. As a suggestion I would say that The Raiders could work, if you extend the turn limit, and add the requirement to land on the planet and/or board any major stations. This does not mean that you need to wipe out the defending ships and defenses or fight the ground troops (wipe out enemy defending AP), but instead come into contact with places that resources are kept (Shipyards, ground facilities) so that you can raid the enemy stockpiles. Overall think a planetary assault mission that is more like a smash-and-grab where you blow a whole in the defenses so you can land and then make off with some loot.
2) For your second question, I think that if the only thing around to defend the planet are orbital defenses, it makes sense that they are all that stands in the way of the enemy fleet. As I stated above, I do not believe that fighting the ground troops would be necessary, although landing would be.
3a) For your last question, there is a definite answer. A raid can fail if multiple (hostile) ships try to raid the same planet (even uninhabited ones). If the fleets have the same speed there is a battle to determine who will get to raid the planet. Such a battle can be lost. There is also the fact that uninhabited planets can be the secret tomb world that Necrons are based on, and raiding one can result in an unexpected battle that turns into defeat.
3b) Also, the reward for raiding an uninhabited planet is 5 resource points, which (from my understanding) is barely worth the time of any ship under your command. It would take 10 successful raids to earn enough resources to build the cheapest escort in the game (the Tau Orca), transporting these goods means another 1+ turns before you can do anything of worth, and a ship (or ships) would be stuck doing this menial task instead of something more productive. There are scenarios where it would make sense to raid an uninhabited planet (a spare transport with nothing better to do), but as a whole they seem to be correct in saying it is not worth the risk.
That is all I can come up with for a response. I hope this helps somewhat, either for the OP or for anyone who comes looking for answers later on.
P.S. I assume you have already come up with your own solution by now, and I would like to hear what you decided on.
