ad1: No middle handles. Only two hooks, one for each side. Your model has some wierd issues with parenting and I couldn’t get it to work properly. But when I removed the curves you have and simply inserted a new one, it worked like a charm.
ad2: The mechanical models make the rigging much easier because the individual parts do not deform. Thus you can simply parent the each whole object to a corresponding bone. No need to bother with any weight painting and you can keep all the modifies in your non-destructive workflow ( which is pretty neat, by the way)
ad3: That’s a tricky one. It seems the movement you suggested is physically impossible.The element with the pistons ( the one with the little horizontal bone) cannot move in a straight horizontal line unless you make the pistons to rotate on each end.
Last to note, as I mention, your model has some very odd issues with relationships. As I tried to move various part, I got deformations, artefacts and glitches. It might be advisable to remove all armatures, parents and other constrains and start the rigging over.
You might want to unlock your object modes for this (Top menu:Edit->Lock Object Modes) so you can select your parts in object mode and bones in pose mode. Then just parent them together. Easy