MFG Ratings what are the limiting factors

I just ran into another head scratcher. When changing from P metrics to E rated tires it is really difficult to find any inflation tables. It took a few hours of research to get a number. I looked at several tire sites and posts all over the web. After much reading it seems that Michelin says to increase the pressure by 15 lbs to have the same carrying capacity as the p metric since I am only using the E rated tire to stiffen the sidewall for less gusy/sway I will run them at 50 lbs

It isn't just a Michelin thing but is a industry standard. P SL tires max load comes at 35 psi even if the max sidewall pressure goes even higher (44 psi). So the LT would need 50 psi to get the same load rating. CapriRacer and Tireman9 have both commented on this issue.

The next question was what is safe for the Rim pressure. It seems that Ford no longer puts the max pressure on the rim So again what is the weak point of the component. I do not expect that 50 PSI will do any harm but once I go above the PSI that is on the door I am in no mans land and I suspect I have violated any warranty that the rims come with.

Wheels just like tires have a max load and a max pressure rating. Ford may not publish a pressure rating on their advertising websites but I would bet Ford can supply you with that info.
The wheels Ford uses on their F150 trucks should have no problems with 50 psi. Thats a pretty low pressure.
I see Ford F150 wheel specs show a 2025-2100 lb load capacity on the 6 lug wheels and a 2400 lb on the 7 lug wheel.
Tireman9 (Roger Marble) tire blog has some good input on wheels http://www.rvtiresafety.com/search/label/Wheels.

I've seen damage and results of other commercial operators and have had my own experiences with using wheels with the wrong pressure ratings. A split bead seat flange was the result on steel wheels and cracked valley's on cast aluminum wheels.