The number changed after the SMS was airdropped.

Question 1: Settings-Information-Sending and Receiving-I can contact you with imessage at the following address (tick the box in front of the email) and "Use when starting a new conversation". Please select your mobile phone number below.

Question 2: I'm not sure. Blue imessage requires two people to be online at the same time. Sometimes the network is unstable and you can only send green messages.