Looking at your list of what has been done posted at the front of this thread I see no mention of a fuel pump rebuild or replacement. It appears almost everything else in the fuel system has been addressed. If it is the original pump with original rubber parts or even a rebuild, if done with a NOS parts kit, that is the first place I would look. The rubber flex line between the pump and steel supply line would be the second.
I know there have been comments that it has not been proven ethanol is harmful to the Packard original rubber components but there has to be some reason all the rebuild places selling pump and carb kits are saying their products are made with modern ethanol compatible rubber. A pump rebuild kit from
Then and Now has had favorable reviews by many on the forum.
If both accelerator pumps came from the same place I think I would try another source. Possibly the pumps that are failing are slightly larger than the originals and the lip is being damaged that way. If a pump with a different style or from another place also has issues then I would think something amiss in the pump chamber is about the only thing that can be causing the problem.
EDIT: Another thing if the fuel pump was rebuilt has been the staked in plug holding the lever pivot pin has worked loose for some and if that happens it lets the lever disconnect from the diaphragm
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