one things pala TS... according to your statement, the same electrical load was applied on a similar montero that was also there at the time you are at the accessory shop... your reading with your montero at load is 12.7, while the other montero reads at 13.7... granting everything else equal among your units....

if the vehicle is still under warranty, try to pursue that path...
vehicle headlights should not flicker.... kasi pag nag flicker it signals that there was voltage drop that occured in the electrical line...
pero before you go and isolate the problem on the alternator, be sure that the battery, battery terminals, grounding, are all ok...

try to put the vehicle in high beam and see if the dome lights will dim... or if the gauges will dim/blink then return to normal...