Well, since upgraded Animate Weapons use a random quantum to cast, wouldn't it still be considered mono-Gravity since you're still only using Gravity Quanta? I posted a deck similar to this, using Animated Titans and Gravity pulls for my attack AND defence, with Otys as creature control. It worked alright, but yeah, Gravity just isn't that great of a mono-deck.
Yes, colorless quantums don't take away from the Mono-theme. So it's a viable option.
Good and simple mono-gravity is..
12 x Tower
6 x Charger
6 x Mercenary
6 x Gravity Pull
Replace one Charger with Titan if you have it.
Perhaps, but covers a lot less bases.