A customer with a Saudi number wants to order from my Dubai page. Will the checkout accept it?

Yes. The phone field carries every country code, and the number is checked against that country's real numbering rules, so an impossible number is refused and a real one is accepted. The WhatsApp confirmation then goes to that number like any other. If the number turns out not to be on WhatsApp, the send fails and Reports → Message history says so in plain words, so you can phone them instead. If you have paired the SMS phone, a message type set to only if WhatsApp fails covers exactly this case. The WhatsApp tab in the docs explains the failure reasons.