You should be able to stop the coil movement by gently bending the support brackets to fit snugly. You can also bend the two rods to make fit tighter. The coil just sits in its nest - there are no other screws or clamps.
Typical starting voltage on a ZW is about 8 volts. Your “D” roller touching the first coil would give you the 8 volts you are seeing.
All TMCC, Legacy and DCS operate between 16-18 volts. Do not exceed that. If you don’t have volt meters in line with each ZW handle, use a volt meter to measure the voltage at the track. Find 18 volts and mark the transformer with a paint strip or piece of tape so you do not go beyond 18v.
Forum member Susan Deats has an excellent website with a lot of information about postwar trains and accessories. There are a few sections on the ZW under “power”.