At 31' and rolling heavy we almost max out our load range, but so long as your front axle is ok with Es then it makes sense to me to have the newest up front.
Tread wear and going flat shouldn't be related, DT should have told you how many 32nds of tread each tire had, and also explained that 3/32 is the minimum DOT allowance. You could use that to judge how conservative/ customer-oriented they were for you vs a sales technique. Also the nail may have been covered in your road hazard warranty if you'd bought it.
What's curious is why 1 side was bald vs the other, especially if both were equally inflated and monitored with tpms. Otherwise my first guess would have been that 1 was low, and so the other took the bulk of the load and wore out quicker.