1. Yes, reload is useful, so is accuracy really because at low level even AA spread can be quite bad. But still mostly reload.
2. You can class KM AA gunners on time, although some people prefer not to class from 1st AA Gunner to RapidFire Gunner because it causes a loss of Acc for no additional Reload. Although there is a benefit in the form of more recruits added every level, so reload ability does grow faster even with no additional ability growth added. Whether you choose to fully class on time or not is up to you, but you can safely promote to 1st AA at 21.
3. Yes, you will. They wont be able to be a part of the crew (IJN sailors may work at half effectiveness but wont be able to operate guns/torps) but using AA boats to level sailors is very common, since not only can it be done easily in GB for relatively higher exp than in blitz, but AAing also provides more experts during the process.
4. The 3 AA guns that come to mind are the Lvl 16 3.46" duals, which are fairly fast firing effective AA guns for low level, the lvl 30 3.46" A version, which is like the lvl 16 but with a 0.8s reload time and slightly more damage, and the lvl 25 4.1" dual which has slightly more range and more damage than the lvl 30 3.46" but also has longer reload time.
the lvl 16 2x 3.46" D is 36
The lvl 25 2x 4.1" A is 32
the lvl 30 2x 3.46" A is 36