1. If DL wants to require bags which fit under the seat to go under the seat, it is well within the discretion for crew to require it and to either require it to be placed under the seat or off-loaded and checked.

2. You are 100% correct that the need to even remotely worry about this as well as when you board is largely a problem created by DL's failure to enforce its own carry-on policy. If FAA did its job and properly enforced against DL for failure to implement DL's own policy, your issue would either disappear or largely go away.

3. But, two wrongs don't make a right. So, yes, it's tough on taller people who fly steerage to be forced to place a carry-on under the seat in front of them and yes, it's easily avoidable, but DL's fail doesn't somehow give you a right.