As soon as a transaction is sent from an address with an insufficient balance, that's when the split will occur.
This is a profound misunderstanding of how Bitcoin works, there are no 'balances' in the system.
I am aware that are no actual balances. I was abstracting this out of the conversation just like a wallet
can calculate a balance to display to the user. The point is still valid: Non-segwit chain vs segwit chain
can show different "balances" on an address, or spent/unspent outputs if you want to be more precise.
And unmodified non-segwit miners will not initiate a split under any condition.
Yes, because "blocks that you consider to be valid but which every segwit-enforcing node rejects"
will be orphaned as the longest chain prevails.
Any rollout of segwit must include majority hash power
No, that is one sufficient condition, it can instead include basically all of the users (in particular, economically significant users). Either are sufficient alone. The users define what are the miners and if the user define mining to include segwit, it does... from their perspective it is impossible to violate the rules, and any miner that tries just stops existing-- just as litecoin miners do not exist as far as Bitcoin users (and their nodes) are concerned today.
You chopped the part of my quote where I said "to avoid a network split". I understand the distinction you make, but many non technical people may not. If all the 'economic users' loved Segwit but a majority group of miners refused to go along with them, there would be a split.