Nick Underhill’s concern with Saints O-Line might be “heightened”

No way. Hurst is barely serviceable at LT last season. Penning will get better as he continues to get snaps. Literally every rookie LT on his class struggled during the first half of the season and eventually got better. Penning is essentially a rookie and will continue to get better and better with every game under his belt. You don't hinder his development and experience for a minimal gain in the short term. Let him take him lumps early on and get him ready for December, January, and possibly February.
I agree in theory about letting the rookie takes his lumps. But not at LT, he whiffs once, and Carr is in the tent. However, I defer to the coaches to figure out if he’s serviceable enough to start.

I don’t necessarily disagree with you regarding Hursts play at LT last year, but there needs to be some stats to prove it. I’m only going off my memory.