Connect MiniDv camcorder via USB C & Firewire or MiniDisplay port
I understand that worst case I'll need a Firewire to Thunderbolt adapter and then a Thunderbolt to USB C adapter.


No.  You will need a Firewire to Thunderbolt adapter and then a Thunderbolt 3 to Thunderbolt 2 adapter.

USB-C is only the connector type, it’s still a Thunderbolt port. 

You can’t use a mini DisplayPort cable because you’d be missing the Thunderbolt signaling.  Yes the, the physical connectors are the same, it doesn't mean all the connector pins are there.  Remember, DisplayPort is a component signal/protocol of Thunderbolt.  In other words, a a Thunderbolt cable will support mDP, but an mDP cable doesn’t support Thunderbolt.