In answer to your questions:
1. Yes, you are missing something.
2. No, you don’t need a static IP address for the Z3, Auto VPN takes care of that.
Regarding the IP addressing the printer will need an IP address for the subnet in the secondary office. Without knowing how your secondary office is currently configured it’s hard to say how best to add the Z3 in. Generally though the approach would be: internet connection terminates on Z3, make sure the LAN addressing on VLAN1 at the secondary office is different to the primary office, enable the primary office MX64 as an Auto VPN hub and add the primary office VLAN to the VPN, enable the Z3 as a Auto VPN spoke, select the MX64 as the hub (don’t select default route) and add the secondary office VLAN to the VPN. Now connect the printer to the Z3 at the secondary office, and also the PC (you no longer need to use the client VPN at the secondary office). And if all that works then the primary and secondary offices should be able ‘talk’ to each other, and so you should be able to print from anywhere on your LAN to any printer at either office.