The Death Star weapon can't destroy Yavin because it is a gas giant planet and not a small rocky world like the 4th moon or Alderaan. It would be like trying to knock a cloud out of the sky with a bullet, so they have to go around to get a fix on the rebel moon.
[Oh and to me terrorists or freedom fighters or whatever are all the same thing from different points of view so I don't discriminate.]
They do (and the Speeders operate within the atmosphere exclusively), but honestly how much difference is a coat of paint going to make? I don't see it as a case of leaving the ships relatively unpainted as it would be a hindrance to its performance.
There was a spoof video on youtube about just that.
However... firing into a gas giant planet... wouldn't that ignite all the gas and create one hell of an explosion that could eradicate everything close by, even the Death Star? I'm sure they factored that into it or... they just felt they had plenty of time... why rush things, when the enemy is no match. Just go in for the slow kill, as only amateurs rush.