Yep, and do you have a drill press / mill to drill holes? You might not be able to get it to fit under there with a bit. You might have to hand drill it.
You don't have to commit to it, you know if you sold it back for scrap, you might only loose $40. Sounds like there are a couple loose ends to the design still.
What's the opposite of gear reduction called? Gear increasion? LOL. I think that's what you're talking about. No, I was talking about using a servo with gear reduction so that the servo spins faster than the ballscrew. For the stepper, instead of gear increasion (I made that word up), you may consider a higher lead of ballscrew. You haven't bought the motors? Great, don't. Just hold off. You may go with the JMC servos for about the same cost.
I have a monster spindle that weighs 100 lbs and I still haven't got it running yet. I've come to realize it might be better to user a 6mm single flute bit running at higher RPMs, with a decent acceleration and speed making light cuts and adaptive toolpaths, you can get decent material removal rates.
What's the weight of your spindle system? You might need a pneumatic counterbalance, be it a low damping gas strut, or two, or something else.
That's a whole level of complicated that I can't help you with. It changes everything in regards to the math too. You're complicating things alot here. Are you calling your gantry the X axis?
My understanding is that people often use the multiple Z axis on routers so they don't have to change bits. To a lesser extent to have the option of a spindle with more torque at lower speed and a higher speed spindle on the same machine. If your two Z axis move independently from each other, then they need to be homed independently and reliably if you were to use both on the same job. This adds complexity to the electronics and your board would need to handle that. I'm not an expert on this subject.
Did you read the Linux_Fan thread to see what he did there?
At the risk of sounding repetative, what you have shown as built would work with a similar design to Linux_Fan's. And his kicks butt. But the choice is always yours to make. The easy thing about his design is that the aluminum plate should come flat enough for your purposes, and it's easier to work with. I won't talk about it anymore.
Is the red line on the graph for your motor? Or do the different lines represent different operating voltages for the same motor? If so, what voltage are you going for?
What is the rotor inertia for these motors? Should be in the spec sheet.