Savy - its time to take some serious action. Follow up with the Senator's office first of all, reply and thank them for following it up and express your confusion about the local office response. Note that the office seems inexperienced in the DV lottery which has a sunset provision each year, and all other offices deal with DV winner as they submit their applicaiton. State that the time limit expressed by the local office is applicable to standard AOS applications, and DV is always processed seperatly because of the sunset provision (state that if you are not awarded your greencard by September 30 you will lose your chance). State that at the rate they are processing applications, you will lose your chance, whereas standard AOS applicants are not under these time constraints. Finally, specify that there has been significant recent litigation over this precise issue taken against various USCIS local offices, where the courts are harsly critical of USCIS not taking note of DV cut-offs and the distinction between DV processing and standard AOS processing.
I would also suggest speaking to a good attorney - I can give you a couple of names if you need (you need to be careful, especially with immigration attorney's, some don't know squat). At this stage what you want to ask the attorney is what they can do, if anything, to get you an earlier interview slot given the DV time limit. The hope is, of course, that the attorney has contacts wihtin the office, or can direct a strongly worded letter to the director outlining your situation and potential action that you will take if your case is not handled appropriately. Additionally, I would ask the attorney whether transfering your case to another office is a possibility, and if, so, whether it is worth the "risk" of the file transfer.
Next, I would absolutely find the name of the director of your local office. I would write to him or her and outline the situation much like you did for the Senator (although dropping the litigation information - no one responds well to threats!). In the letter I would include information about USCIS policy on DV processing - this should be readily available on the web, do a google search to identify specific policies and refer to them directly and include a copy.
If you need help with any of this, let me know.