I believe that P3D is equivalent to FSX + Acceleration, so there should be no problem. What could happen is that some add-ons will search for the Acceleration pack in the sim's software and that search will fail in P3D.
I know you had a few problems with FSX but couldn't you install it temporarily so you can pick the missing planes, missions and such out of it for use in P3D then uninstall it You wouldn't need to worry about aliases then at least.
Its a shame its not possible to open the cab files on the FSX and Acceleration discs like you could with FS9.