Just for your information (and correct me if I am wrong someone) -when you hard reset you are not going to "lose your flash". A hard reset will not revert back to the stock Rom. It will revert the custom Rom back to the exact same way you installed it. But you would lose any customizations you have done to the custom Rom.
As far as a fast way to delete all appointments and contacts -do you use Microsoft MyPhone?
http://myphone.microsoft.com/mkweb/S...1033&mkt=en-US
Put the cab on your phone. Install MyPhone. Run it. Sync all your stuff from your phone to MyPhone. Then on your PC go to the MyPhone site (link above). Log in with your Windows Live ID (I think -I am always logged in). Select calander, or contact, or whatever you want -then you can put a check mark that checks them all. Then you select delete. When you are done deleting everything -go back to your phone and sync your phone with MyPhone.
I made it sound very complicated. Sorry. It is not complicated.