Ok, I might not have the final solution to the problem but this should lead you down the right path.
In the JDParts indexes I found there are multiple 54" mower decks. It depends on how you search. Based upon your signature you have a GX345 and that is different from just a 345. The previous parts diagram posted appears to be from the X345 as shown on this attachment:
If you look at the photos supplied and compare the parts diagram #28 is not the same shaped part as the photo.
If you examine the diagram found by searching for just 345 as shown on the attachment below:
the parts diagram #21 on this drawing does match the photos.
When I went searching through the index I found multiple occurrences for the GX345 54" mower decks. I did not take the time to compare them to see if they were the same or different but the titles indicated multiple serial # ranges.
In summary there are several possibilities that you will have to sort out with your dealer. If the pulley assembly has been replaced previously it more than likely was the wrong part for the model of mower you have. If all the parts are original to the mower then you have a mower that came from a 345 not a GX345 or does not have the correct serial # range applicable.
Good luck in determining who was at fault. For the members of the forum this is where you can eliminate some of the problem by accurately referencing the complete model and serial number and manufacture date when purchasing replacement parts.
The list of new parts you are purchasing may be necessary but in my opinion are not the correct parts to solve the problem you are having, with the excpetion of the PTO sheeve. Once you get the correct parts diagram the first part I would replace is the double sheeved pulley.