How will BU run the network without forking it?
Same way it is today. My node is a BU node. No fork so far. Observedly.
You seem to be seeking demons where none exist.
BU nodes today don't enforce BU rules, they enforce Core's rules. Because BU hasn't forked the chain yet. BU can only run it's rules when it's the dominant mining software.
Or are you trying to pretend that more than 1 set of rules can be run simultaneously? This is so boring, you've tried these nonsense arguments before.