how to electrically isolate a PCB from a heat sink

Typical solutions I've used are either Kapton tape:

Or Sil-Pad:

Sil-Pad 400 is a composite of silicone rubber and fiberglass. The material is flame retardant and is specially formulated for use as a thermally conductive insulator. The primary use for Sil-Pad 400 is to electrically isolate power sources from heat sinks.

If you use a sil-pad the screw will still electrically connect the componet tab with the heat sink. A solution to this is to use a shoulder washer to isolate the screw and the tab:

The bracket of the lights is most likely grounded by design.

I can't think of a reason why a dimmer module and lights cannot both be grounded to the motorcycle, and not work except that the "IQ-170 Intelligent Lighting Controller" dimmer is actually not so intelligently designed. The geniuses who came up with this certainly did not have an IQ anywhere near 170. They didn't consider lights that are grounded to the chassis via their mounting hardware, and rely on that for heat dissipation too.

A properly designed dimmer will control the V+ side of the light, and not require its ground to be lifted, which, as you can see, creates difficulties.

You have a very good reason for returning this thing: it is technically flawed. I would contact the company for technical support. Perhaps the module can work on the other side of the lights.