The contactor is inside the "doghouse" area of the monolith, the block at the top rear of the case. The HV fuses are on the left side of that doghouse. Do not open the doghouse without a fully-electrically-safe workspace set up, because there is live voltage in there at all times from the battery pack.
Definitely insulate those leads properly as you disconnect them to avoid any severe faults if you forget while trying to test by powering up the bike. Really, just don't power up the bike, but cover the leads anyway.
It sounds like you have definitely narrowed the problem to the controller. Unfortunately, only dealer tools can properly configure a replacement Sevcon controller for a 2017+ model year Zero.
I'll emphasize that any further teardown you undergo will not endear you with your dealer or Zero, especially if you don't conduct it properly. This is warranty-voiding activity, really, and potentially fire-starting, equipment-vaporizing, and maybe injurious or lethal outcomes.