We can use kinda forward+ for GL2+, so it's not so far.
And deferred rendering prevents MSAA, most everything in graphics is tradeoff.
Most modern graphic systems in games don't cast shadows for dynamic lightings, might do only for few (like 4-8) nearest to a player lights.
Not really, you more wanna say too much to review at once x)
Profit of the instancing is pretty tricky, it highly depends on a scene. Look at Yves measurements for deep forest. For obvious reason the difference isn't so big about ~10%.
It depends on how to add lights, usually the number of fragments doesn't scale with the number of lights. And there are a bunch of pretty old techniques to reduce amount of rendered fragment.
In terms of the game implementation it has most needed features, but in Atlas we can't edit all its parameters yet, only a part of them. But it's possible to edit the map file or use JS to control all parameters.