1) Can vary, but melee DPS has the shortest queue imo. Most three mans run tank/ranged/healer leaving the melee slot open for randos. Ranged DPS has the worst queue after the ranged/melee DPS bifurcation change.
2) Queue 30/40 solo. You can pull a 40%+ win rate generally. I vastly prefer 40 to 30, but the queue times can be longer if you only queue for 40.
3) Varies widely depending on job. Getting used to line of sight mechanics is a big universal one to avoid damage. Don't dash to the middle at the start unless you have a reason to. Sort targeting priority with your teammates prior to gates opening. I'm sure others will be able to provide additional suggestions.