Yes! If you leave the default channel enabled it will join the public meshtastic network. But you can run eight channels at once, and the others can have different encryption keys.

The default setting is also to forward any received packets to the mesh, even ones it can’t decrypt.

For a good compromise, use Meshtastic. Long-range radio mesh network for texting that phones can use instead of cell networks.

No idea about bike licenses.

High power radios in the hands of people who don’t know what they’re doing can cause a lot of issues. FRS/GMRS radios under 2 Watts in the U.S. don’t need a license. Licensing is a way to make sure people using them know how to do so effectively, and provides for stiffer penalties if someone unlicensed causes issues.