Not sure why you seem to be trying to re-invent the wheel with every aspect of your machine.
I don't have any actual experience, but I don't think you'll be happy with direct drive. I have a feeling you won't be getting very smooth motion, especially at lower speeds. The mechmate guys like to use 3:1 reduction, and they say that a larger pinion than 1" gives noticably smoother cuts than a 1" pinion. So I'd think that a direct drive 1" pinion would be much worse than a 3:1 1" pinion. Also, I've heard that larger motors aren't nearly as smooth as smaller ones, especially when you get into the 4000oz range.
You would think that if direct drive was a viable option, someone would be doing it?
1) Don't know
2) I would think that 4000oz would be much too large for a Gecko. Gecko's have a switch for a Nema 42 mode, but I think they are best suited to smaller motors, say, 1200oz or smaller.
3) Don't know.
4) My guess is absolutely.
Why don't you just buy the CNC Router Parts drive components. They're a proven design that works. And when you start adding up the cost of 4000 oz motors and drives, I don't think there would be much of a cost difference.