You certainly have more experience than I do with Maker API, and I'm just shooting in the dark with ideas to track it down.
Just to be clear, I think you said that other commands in the same Hubitat driver are working from your Crestron setup through Maker API. That would seem to indicate that your headers and general request structure are fine and that something is getting mangled with this specific request.
If your latest shot at it still fails, maybe try this: Instead of the date string with dashes, try sending a flat string of any value. I'd like to see if you can get inside suspendZone and fail there (due to the date not parsing) rather than the 404 from (I assume) Maker API itself.
FYI, here is some info about using commands with multiple parameters: Maker API New Features
