Osmosis does not heal. It the name of her passive, by which she generates Osmosis Stacks. Wellspring is her healing ability, and it consumes Osmosis stacks. The greater the stacks, the greater the heal. Wellspring is necessarily a single-target heal.
All Battleborn have more than 800 HP.
A good time to use full stacks is when someone can utilize it all or most of it, but before they are dead. This varies throughout the game (early game you can only heal ~600, but late game I’ve dropped a max 2122 on someone). In general, it’s a good idea to use it on someone below half health.
Not to my knowledge. There may be a bug that causes SD gear to buff it like AD gear buffing Miko’s heal in the past, but they patched that Miko bug, and I expect that they would have patched any similar bugs like it.
No. She only heals her single target. The only multi heal she has comes through her level 1 helix.