Both tested on version 1.2 of the game.

You can reload weapon even when you have less than 15 AP if you use reload hotkey (9).
There is a way to make a Devastator Squad with all marines equipped with heavy weapons. Basic thing is that 5th marine in Devastator Squad always gets Heavy Bolter equipped by default.
First of all you need at least one dead marine (empty slot).

Remove one marine from the Devastator Squad (to have four marines in a squad).

Enter ‘Configure Squad’ screen.

Give heavy weapons to two marines and return to main hall.

Enter the ‘Select and Configure Squads’ screen and assign one marine to Devastator Squad.

Enter ‘Configure Squad’ screen. You will notice that 5th marine has a heavy bolter. You can change it for any other heavy weapon. Return to main hall.