To be honest the diodes arent on the wiring diagrams, shouldnt really make a difference, but if its working now it goes back to the legendary saying "if it aint broke, dont fix it"

Nah those readings are fine, thats pretty good actually if your still charging at 13.1v at idle with full load (i bet u didnt have the cooling fan on though did u )

It will increase when you raise from idle anyway, sounds to me like the jobs a good un