Ditto last two posts. Handheld for ID and "pie"ing a room, weapon mounted once a thread is identified and engaged. Weapon mounted only makes me nervous in that I must laser the threat while identifying (or at least come close to doing so).
Training in low light is essential. Professional training is a super plus. You would be surprised how different it is than well lit. But lots of "Oh, Yeah" when done with an instructor behind you. Disorienting and high stress. Also keep training weak and strong one hand shooting. Very handy in quick, close, dark encounters.
Good sights visible in low light make a difference, too.
Good question. To actually address it, I like a modified Harries, with the weak hand holding the light and kinda cammed into the thumbswell of the strong hand master grip. On the left side of the grip (I'm right handed). It steadies the shot, more than supports the strong hand, but works OK for me.
Practice drill (even in daylight) is gun in hand low ready, light in off hand pointed downrange, pistol up, one shot, bring weak hand and light to left side of grip, a timed pair to a small part of the target. Repeat as needed.
Steve