How to sync two or more collocation database server?
Suggest you have multiple instances in the central server.  It could be a single installation of mysql, but separate data directories and my.cnf and Port (or use VMs).

After doing that, you can make lots of pairs of "Dual Master" setups where either Master can be written to.  And they can get into serious trouble with auto_increment keys, unique keys, and other things that plague M-M.

No, I really think, you should start over on your 'requirements'.  Currently you are asking for a maintenance nightmare.