That's why at Wormhole we chose to use 100.64.0.0/10 (and actually most of our customers only use the default first /24). It's reserved for carrier grade NAT, thus unlikely to interfere with anything you or your providers use :-)
We offer a similar service to ZeroTier, but based on SoftEther. Needless to say, we LOVE ZeroTier, they've got a brilliant product. Keep up the good work!
Yes. Sort of. IPv4's address space is too small so it's a nasty hack people use to avoid conflicts. It doesn't hurt anything but if those blocks ever get advertised in BGP you have to renumber your network.
Also keep in mind that we only use that range on our test "Earth" network. You can use any IP range you want on your own networks. ZeroTier is layer 2 (Ethernet).
BTW, it's not uncommon for cell networks and ISPs to use DOD IPs for hidden layers, leading to some interesting outbreaks of paranoia when people spot them in a traceroute: "why is all my traffic being routed through the DOD?!?!?!" (No, it's not.)
