Re: Persistence between MAC changes
The MAC address doesn't change at random times. It changes per network/per connection/when scanning (depending on the implementation). Once you are connected to a network there is no point to change the MAC mid-session.
If someone can track your cookie then that's another kettle of fish. Just by being connected to the network should not make your cookies to another site visible (if it does, you have other, bigger, problems than handset identification)
If a MAC collision occurs - tough luck, one or both of the connections will probably stop working. You have to choose your battles.
DHCP - some implementations (windows 10) will use the same MAC when reconnecting to the same network so this is solved in that manner (this allows a store to know that the same user went into the store at certain times, but at least it hinders tracking across networks) - another case of Privacy vs Convenience