Will I get the OTA update if I manually put 2.2 on DX? If not can I put my DX 2.2 back to stock?
It's hard to say whether or not you'll get the OTA if you're running the leaked update. Some say yes, some say no, but the truth is no one really knows for sure.
And yes, the phone can be put back to the stock 2.1 OS by flashing it with the sbf file. There are several threads here that discuss exactly how to do that.
My advice is get the sbf file before you upgrade to be safe. Then at worse you can always go back to 2.1.
2.2 is so nice in that wi-fi and vpn finally works, as well as gps and better batttery life that it is worth upgrading now. If you can find the 2.2, you can fine the sbf
i would guess most likely, but even if not once the final ROM goes OTA, i bet you can just download and install it yourself just like you do now with the leaked version - the final version will be all over the web so fast.