Oh crumbs. Here we go!

[Jonathan]

From the picture, unless it's a twin start screw, it looks like you got RM1605 for the longest screw, not RM1610. Assuming the ballscrew is 1150mm long, as per the original post, this will limit you to about 8.5m/min due to the critical speed. That speed is plenty, but I doubt you will get it with 3Nm motors unless you have a 2:1 ratio between the motor and the ballscrew to get 10mm 'effective pitch'.

20mm is plenty thick enough as a back plate for the Y-axis, so long as you have some box section or extrusion bolted to it to increase the strength parallel to X and the torsional stiffness. It will be much more rigid if you have the ballscrew directly between the rails, so on the same side is good.