The fact that you have to pay $20 to someone else for your trainer to come on property says to me that the BO is dishonest and annoying. But I'm the kind of person who likes to have my horse in my backyard or not more than 5 minutes away so I can over see all of my horse's care. I don't like full board situations.
So if you leave for a short term board somewhere else till there is an opening wher you truly WANT to be- you may find your self responsible for at least an extra month board ( if you are expected to give 30 days notice which is the norm) I guess I would be up front with the temporary barn and see what you can arrange. I do think it would be extremely unfair to the temporary BO for you to indicate you are moving in for the long term... and then up and leave on her. SHe may have turned aside other boarders for the spot you take.
ALso.. consider that your horse will have the adjustment period with one new herd... just get settled in and then have to start all over with the other.