Although I had the duct leaks identified and sealed, including my one return directly through the wall under the ahu stand, one contractor who later did a Manual J and further leak testing (in preparation for his new equipment quote) determined there is still a 17CFM leak at the return that needs to be sealed. This space under the ahu is "unfinished", that is, the wall studs below the top of the stand are not covered by drywall. The contractor who sealed the duct leaks attempted to "seal" the space with ductboard, but apparently his efforts were not 100% effective. I would guess the 17CFM of leakage would allow attic air to leak into the return.
With a less restrictive MERV 8 filter, the suction from the return would pull more air in from the conditioned space in the hallway where the return filter grill is. With a more restrictive MERV 10 (Honeywell 4") or MERV 11 (1" pleated) filter, suction from the hallway would be restricted and more air would be pulled in through the "leak" from the hot attic.
Given the poor, but constant, condition of the condenser coil, this added heat load might just be enough to trip the high pressure switch.
Does that make sense?