They are not obligated, but if you get a negative result, you're obligated to declare upfront. Otherwise,  there's always a chance they'll find out later and deny your claims. That would be worse after having paid lots of premiums. I'd prefer to declare everything up front.

I'm not an agent, but no. Simply because you did not disclose all the information they requested. In essence, you were trying to cheat them. If you had disclosed full information, they might have added loading, exceptions, or even rejected you outright.

Maybe someone else who has experience can let you know.

Of course you know! You signed the declaration forms when you bought the insurance.

Eg. http://www.etiqa.com.my/Document/1471835916033 All insurers have similar forms.