I recently listed a home and in 10-12 days or so had three accepted offers come and go! And about a dozen agents called with "please let me know if this falls through!" yet not one of them wrote a back-up offer. Hadthey, they would have had the home. In the end, one did become the ultimate buyer but any number of agents and buyers could have had that position.
My suggestion is find out if you can write a back-up offer and do so. If agreeable, write it to the latest terms the seller will accept.
I'd ask your agent to do some research into this... you are his/her client and for many of us it is an honor to do special things for our clients.