After it has been running a while feel the driver heat sinks and the steppers they can be really warm but hot might indicate too much current and need to be dialed back a bit.
1)In theory 2 times as accurate but really do difference, I feel they allow you to run things with more current = more torque. 16T pulley I believe also increase the torque by 20% over the 20T. I did spend a little time making my choices 🙂
2)Agreed 100% I did my best to choose readily available materials and hardware world wide. Not an easy design task….
3)A very likely cause. There is a printable version, should help. http://www.thingiverse.com/thing:1105029
let me know how its progressing please.