If I pre-order HOTS now, can I give my beta key to someone else to use with their account and then when the game is released properly still play with my own account?
In my experience, yes. In my case, I ordered from Amazon, and they emailed me a beta key. The beta key is not linked to anything yet, so anyone can use it. To attach it to a specific Battle.net account, the owner of the account just has to log in to battle.net and apply the key, which will then give them access to the beta.