I was able to coordinate FF tickets with a Concierge Collection exchange to Kapalua Bay in Hawaii. I was concerned about all the same things you are, but everything worked out fine. I'm not real sure why it takes so long....I'm suspecting that someone in some other office than where the customer service reps sit actually negotiates the exchange and we know what happens when you start moving papers from one desk to the other.
It's been about 10 months since I actually made the reservations, but if memory serves, I think I jumped on the tickets, since Hawaii ones are supposed to be hard to get, and figured that I had several options (concierge, exchange) when it came to the hotel. If it helps, I've exchanged with the CC twice and I seem to get the feeling that you are not often turned down, unless you are trying to get an extremely impossible date at a very popular location. Just a feeling, though.
I'm not sure if this will help, but I looked at switching segments of my flights around to try to arrange to have two paying tickets at a good fare on the same flight as me. My FF's were with American, and they did not charge me a change fee, because there were FF's. You might want to ask the airline about that. Also, when I was looking at changing, it seemed to be relatively easy to get FF tickets to California...it was the CA to HI segments that I was never able to change. Beverly Lynn on the Transportation Board may be able to answer some questions about the airline policies.
Hopes this helps a little.