That depends on what you want to happen, and on how the recipient has set up Merlin on their device. If you decide to 'Save and deliver all instant messages' then when the recipient signs in, they will all be delivered. However, because some people find it annoying to receive old messages, they may decide to 'Discard instant messages sent to me before I logged in', in which case they won't get them. The best thing to do is to send instant messages only to people you see online.
Yes. You can completely remove messages you have sent up to one hour after you send them. If you remove a message to someone who has not yet received it, it will not be delivered. If you remove a message to someone who had already received it, they will see the words "message removed".