I also wonder if they'll be aggregating multiple purchases together over time to trigger the threshold.
This is almost certainly never going to happen.
The process for confirming KYC information is to verify the information prior to the transaction being initiated, so they will have no way of knowing if your history meets the threshold to require them to have verified your KYC information.
they will via account login. this "continue as guest" button won't be around forever. i'm sure having an account will be mandatory at some point. so you'd have to juggle multiple accounts with VPN and hope to dodge their algorithms for triggering KYC or detecting multi-accounters, which would probably work at least for a time. but they will also be clocking users with other conventional analysis to mitigate compliance risk: exchanging customer information with merchants, logging IP addresses, browser analysis, all manner of other leaked PII.
it's not like they need to prove anything beyond a reasonable doubt. it's the opposite. they will just have various forms of risk analysis to cut off risky customers who seem to be dodging their terms. multi-accounting will definitely be against their terms once accounts are mandatory.
will it be possible to get around this? sure, with some common sense and effort. but most people are sheep and will just voluntarily give up their KYC.
My understanding is the prompt will only appear if the transaction size exceeds the threshold mandating KYC collection. I understand you can create an account that holds your KYC information so all you need to do is login to proceed with the transaction and the continue as a guest button will allow you to enter your KYC information on a one time basis I dont think using this button will allow you to bypass KYC.
BitPay will only collect KYC information to the extent that they have to. Every time they collect KYC, they might not be able to close the sale which means less revenue for them. You might be correct in that customers may eventually have to create an account to buy anything from a merchant using BitPay, but they have given no indication this is coming. I dont see any real advantage to requiring customers create an account, except for compliance with regulations, so I dont see them forcing users to create an account until their regulators strong arm them to.