I am guessing that you then made changes to improve the DLC and fix bugs.
If you are using the "fixed" version and trying to port it to the PS3 then I doubt the changes made will be compatible with the PS3 version.
However it would work for the PC because the 360 are much similar to each other than the PS3 and PC or the 360 and PS3.
So if you are porting a non original version of Dawnguard over to the PS3 and you are having major issues I declare that the original "Beta" version be ported to the PS3 and the "Beta testing" process repeated on the PS3.
Again I don't know anything about programming but if this detail has been overlooked then it might be the cause of the major issues. I know that the PS3 and 360 versions constantly have different bugs.
The water physics glitch for example was a major issue for PS3 users that had to be self resolved. I never heard of a similar error on the 360. These common differences is what leads me to believe that if Dawnguard is to be ported over it must be a different version than the one currently available for the 360 and PC because the bug fixes made in the current version will cause incompatibility with the PS3.