In any event, barring a written agreement to the contrary, the listing broker (who you call the Selling Agent) certainly can contact other interested parties.
On another note, if you are using FHA financing, and the roof is in really bad condition, appraisers seem to be noticing this and stating so in their appraisals. The bank will require it to be complete prior to cloing. So this is something that your agent should have used in helping you to negotiate. By needing a roof, it must be disclosed and then begins to limit the lending options, your mortgage broker should be able to answer this.
I also suggest you have a heart to heart with your agent and if you feel that you have been pushed, speak directly with their broker. This is a happy time and you should not be feeling remorse prior to the closing at all.
It is fair for the seller's agent to contact them and is actually doing what is in best interest of the client(seller).
As far as being pushed into the agreement, hopefully you put in a sound offer at the beginning and are happy with the $500.00. Time is of the essence with any transaction and it is important to take note of the stated times. This is not 2005, but some homes are selling quickly and you should be happy that you are getting what you want, not losing out to someone else.
It definately can make a difference that they are in the same office. Unfortunately you have no way of know what type of relationship the two agents have. They could be friends or hate working with each other. You just don't know. My feeling is the listing broker and the selling broker should be completely different, Not all agree with me but at least you get sole representation.
Duncan Real Estate