wow that really stinks! first of all, i think you need to do what YOU and FI want to do and try your darndest not to let others influence your choice.
you say the family is not willing to help with an AHR or anything, so im assuming you and FI are paying for the DW yourselves, which means you have no worries of anyone "forcing" you to do anything you dont want... your $= your way!
honestly, your reasons for picking a november wedding FAR outweigh FMIL's reasons for having an august wedding. maybe you could draw up a list of pro's and cons for each month and present it to her so she realizes that you are making a very rational choice? i know sometimes when you try and tell someone something they only hear the parts they want, but if its down on paper, how can they argue? (dont forget to include that $ help you want to give SIL for the unpaid week as a PRO!)
anyways, i really hope you keep the november date for many reasons, but the biggest one is i'd hate for you to have any regrets later down the road. every bride wants happy memories of the entire wedding, and the best way to do that a lot of times is to do what you want and need to do because its YOUR day (not FMIL's hehehe)
either way, keep us posted!