Well thats sort of the thing...with Visa and MC, there is an extra layer or two of the bank or Fidelity Information Services. With Amex, they own the full stack end to end.
Nope. The merchant can choose the level of verification - in some cases, like copying the card with an imprinter [1] or running phone transactions (yes, that is possible - it's called MOTO [2]), it's obviously impossible to check card limits.
Downside of CNP transactions is, the merchant is fully liable for anything from fraud over chargebacks to exceeding limits.
And then you got card-present transactions but the network connectivity is down for whatever reasons... been a while since I messed with that, but at least for German cards you could configure the terminal to store the account details for later submission when connectivity was restored.
[1] https://en.wikipedia.org/wiki/Credit_card_imprinter
[2] https://docs.adyen.com/point-of-sale/mail-and-telephone-orde...