The question is if Boeing knew that the manual trim forces were too high for an average pilot..
I don't buy this average pilot stuff. If the trim can only be operated by a pilot with average strength or higher, in some circumstances, then assuming a normal distribution of strength then 50% of pilots will have less than average strength. If strength greater than that deliverable by 50% of pilots is required to trim in some scenarios then certification should specify the minimum strength required by the pilot and a relevant method for evaluating a pilot's fitness to fly the type.