As for passengers disembarking, I just measure 2" from the entry points looking straight down at the vehicle, then place the passengers at the corresponding spot on the ground.
Depends on the rules for the specific skimmer.
The Storm Raven rules say anywhere within two inches of the base. So you could have a dreadnought unload up to two inches away from the front of the Storm Raven. Not sure about the wording on the other skimmer as I do not run them.