It must be the case that you were talking about OM too much to this relative of yours. His name must have come up too much when you were talking about all the things you did with him. It still seems like it would be a leap for someone who has never seen you with him, to come out and accuse you of an affair. When someone sees you in person, they can watch how you both interact with each other, and catch your vibes so to speak. You must make it a point to bring up OMs name when you talk to this relative. Also could be the way you talk about him.
Maybe OM does not care that someone suspects you two because his marriage seems to be coming to an end and therefore it would not really affect him in terms of breaking up his marriage. However he could show more concern for you in that it could really turn your life upside down if too many rumors and accusations started flying.
Don't know what to think of the office rental, could go either way on that one.
I wonder what HE would do if he became free. You said you do not know what you would do, but I wonder about him too. Would he continue to stay in touch with you and flirt with you? Would he try to get closer to you? Would he suddenly back away? Would he try to pursue some 'available' women? Maybe he would just need time and space to be by himself for a while.
What do you see from your husband that makes you think he is noticing a change in you and that he is nervous?
So yes it would probably be good for you to stop mentioning OMs name so much when talking to this relative and for that matter anyone else unless you know you can trust them and they know whats truly going on and won't spill the beans.
Love 
