Congrats Dan, I'm glad your repair went well. To be honest, I'm a bit concerned that the sponginess of the mouse pad is going to allow the board to shift somewhat. Hopefully it won't be enough to cause you any hassles. I would definitely recommend using a solid material for the spacer/shim. I know it's not easy to find something 2mm thick to make one from. I have a hard plastic automotive mud flap that's 2mm thick. If you like, PM me your address and I'll send you a piece large enough to make two from (it should go in an envelope).
When I first installed the Omron 4005 replacement switch, my main concerns were that it end up at the correct height and be strong enough to support the pressure of being pressed over and over again over time. As you already know, this switch is not the correct height nor is it designed to be surface mounted (it's a through-hole switch), so it takes a little modification to make it work in the Zmax. I wanted those plastic pegs that protrude from the bottom of the switch to rest on the circuit card so the stress wouldn't all be put on the legs (legs might bend or bow?). I decided to sand them down until the overall height of the switch was at 5mm (same height as the original Sigelei switch). I straightened and re-bent all 4 switch legs out to the side so that all 4 legs including the sanded plastic pegs made contact when placed on a flat surface.
Here's what I did:
1. Fully straightened the switch legs, and then bent them out to the side (90 degrees) so the switch would fit in the jaws of a micrometer set to 5mm.
2. Sanded the plastic pegs back and forth over a piece of medium grit sandpaper laid on a flat surface (tried to keep it level as I sanded).
3. After several swipes, placed the switch in the jaws of the micrometer. If the pegs were still too long, repeated until satisfied.
4. Trimmed the legs down in preparation for final soldering to the circuit board. The old switch is useful for getting a length comparison.
In this pic, I hadn't trimmed the legs shorter yet:
