Either Dan or Gilly is fine.
I left out one important note 2 posts ago where I was quoting stuff:
In Body and Accesories Chapter 6.1, which is the Closing Assist chapter, they have a test for the trunk pull-down also, it's test step 15, on page 33/9. This indicated that the early cars did use CA for trunk pull-down. They also have that same note about the "starting with 118121" it's pulled down by the PSE in parallel with the RTG.
I'm not trying to be confrontational about this. Yes, I do believe the problem is the PSE for several reasons. #1, if it were old enough to be the CA pulling down the trunk latch tongue (pre-118121), it makes no sense why it would do it in reverse, since the PSE is in charge of the reverse rods. Also "Jere" hasn't replied to posts here for awhile, but since he stated the car is an S500, it probably is a newer car, as cars this old (pre-118121) would probably be badged a 500SEL yet.
On Warrens car, I believe that a replacement of the PSE would have repaired a problem with the trunk pull-down, as we have been discussing. Maybe someone referred to it as the closing assist pump, but i think they meant PSE pump, may have used a term more understandable than "PSE". The PSE pump is more understandable if described as the "central locking" pump. As it is, I think PSE can be loosely translated as "Pneumatically Slaved Equipment".
On Old South's car, the more I think about it, the more I think it has to be the latch itself. Possibly the microswitch, but since you observed the cam not properly contacting the switch, make more sense it is a latch problem. You should be able to monitor this just by closing the latch with your finger or a screwdriver.
If it happens quite often, it would be easy to diagnose by simply monitoring the "actual value" screen on an MB tester (HHT or SDS) in the PSE control module.