Brackenridge was brought in because he's an allstar safety and a proven game changer. Sadly he is no more.
You hit the nail on the head that there is no backup for Emry, at least not a NAT backup. Which brings us back to the Foley trade. They had Foley to start at end and had his quality NAT backup David Lee as well. When they traded away Foley for Emry they had no NAT backup for Emry, at least not one that CC trusted (Hurl/Regimbald/Antwi).
McDougall does have a backup (Carter or maybe even Webster) but he was not planned to be a starter, TBrack was supposed to be the starter. McDougall only started week 1 due to injury of TBrack. Connop was supposed to start (backed up by Agbaje - now Tennant), remember that was the plan, they tried to hide Walker on the 6 game with a fake injury to do this. Then after week 1 they realized that Connop wasn't going to cut it so they reactivated Walker. The problem was that they couldn't start Walker because MacDougall got f'd up on the opening kickoff so they've been stuck with Connop's birth certificate ever since and Walker can only rotate in on certain packages.
So this whole mess is all because they failed to backup their NAT starters with NAT backups. CC and Taman share this as far as I can tell.
If they had not traded for Emry Foley could be the 7th NAT starter and if Foley had gotten hurt just like Emry David Lee would be doing a solid job at end RIGHT NOW!
Right now the only realistic way to start Walker is to start Carter (or possibly Webster) at safety and CC is reluctant to do this. Either because he is afraid of too many rookies in the secndary or he wants TBrack's playmakeabillity that has simply not shown itself and not looking to any time soon.
And after all of this, the defense is NOT HORRIBLE. It plays very well for very good stretches then has bad lapses and comes apart at the worst times. The interception put the ball on the 5 yard line and they got the stop for the FG. They came up huge there. This is still very fixable. Great to see Maze playing MILES better than 2 or 3 weeks ago. Jackson might be back this week. Perhaps LeGree might be soon. Hollins is a wonderful surprise. Doughty has been solid, Knox a strong presence. The line is a different beast with Walker in at tackle. I still like where this can go.