Google Nearby API is proprietary and doesn't do mesh. Normally WiFi direct on Android gives 192.168.49.1 to all group owners, so you get an IP address conflict if you have more than one group owner on the network. I found it is possible to to get around this using IPv6 link local addresses.
I have now gone and built an open-source library that implements this:
https://www.github.com/UstadMobile/Meshrabiya .
In testing I can get 300Mbps using a 5Ghz network on neighbor nodes, 150Mbps over multiple hops.
I have checked out Briar, Bridgify, Qaul, and others - Is there anything else that can setup a multihop mesh network on Android over WiFi without requiring a rooted phone? Anything else out there to interoperate with?