Conflict confusion as follows:
1a. psi should be as close to max as possible to insure best overall mpg,
1b. rolling resistance of 51psi on your rig with moving mass in bed and on tongue will be the best overall for mpg and to assist in removing "roll" or spongy cornering
1c. max pressure listed on the sidewall has NOTHING to do with fuel economy. Rolling resistance is mostly controlled by the materials the tire is made out of
2a. skip whatever the placard in the door jamb says.
2b. the pressure listed on the vehicle tire placard is applicable for ALL tires in that size, regardless of who makes the tire. You may find that the ride or the handling is different, but that is NOT because of the max pressure. It's due to the way the tire engineer designed the tire - and those things do not affect that load vs inflation pressure relationship.
3a. 51 psi tire will get poorer mpg at 40psi than a 44psi tire
3b. 44 psi vs 51 psi shouldn't play into which you should be choosing.