Though I am not sure what was the intention. But while you are oversea and getting a local sim with data. There is no need to "re-validate" the phone number. Just put in the sim and Whatsapp will just work as it is. Whatsapp recognises the device and not the phone number. e.g. Even when you are oversea, with the sg sim removed, with wifi, you will still be able to get your Whatsapp messages.
People does "re-register" phone number is usually those that moves to another country and need Whatsapp to move their sg phone number account to the oversea number. That's when you need to do that. Else if jsut temp situation, just put local sim will do.
You mentioned that you also don't want them to know you are oversea and can receive messages? The only way you can try is go to the Whatsapp app's Setting>Account>Privacy section, turn off those Last Seen, Status, Read Receipts. Then people will not be able to tell when was the messages was delivered to you or if you have read it. My description was for iPhone, other OS phone, not sure as i have long didn't use them. Also I cannot 100% sure if that works as I have never turn off any of those before since I used Whatsapp.