I'll admit I did not read your *entire* post.
I carry a gun every day, for more than 13 years. Also have been to numerous schools.
I can say, without pause, that "mexican carry" is as bad an idea as pocket carry (without apocket holster).
It is NOT secure, period. How can wondering if my pistol is slipping or going to fall out all day be confortable?
It's a sure way to either
a) drop your gun
b) shoot yourself while carrying (no triggerguard protection)
c) shoot yourself while drawing.
And, IMHO, since it is NOT safe it does not convey a good image of carrying.
You asked for them and those are my opinions. And this is in no way intended to sound snide, but based on your opinions you posted I would HIGHLY recommend some professional training. I think real world practical study would convince otherwise on the idea of not using a holster.