Well, 389 yards, which would reasonably round to 400. Particularly if you add back on the -2 lost from kneeldowns at the end, it's 391.
But I find your take to be strange in that you get into nuance on one issue (yards) but you offer zero nuance on another issue (defensive injuries).
Yards:
If the Lions had pick-sixes, punt returns, basically Chicago Bears-style football getting us all our points, then nuance about yardage matters. But that's not what happened. Every TD was from the offense, and just one short field on the DK fumble. Every other drive they went the length of the field. What's the yardage actually matter? It doesn't, not really. It's strange that the Lions were something like -130 in yards and yet won by 2 scores, but the reason is pretty simple. Every Lions drive was either all the way into the endzone, or else a 3 & out (or a safety). I don't think they gained a single first down on a drive resulting in a punt. The Seahawks I don't think had a single 3 & out. Drives that get one or two first downs and then punt are generally not considered successful, although can be in some circumstances - but not when you're constantly down by one to two scores. Because if a drive into the red zone that results in an INT and a touchback isn't a successful drive, then how is a drive with three first downs and a touchback punt any better? It isn't, unless morale really is an issue. Essentially, the Lions made the most of their opportunities to an extreme degree, and the Seahawks had moderate production which sometimes led to no points. This is just football, man. I had a decade of people saying my QB was Stat Padford, racking up meaningless yards and TDs in losses, and now suddenly when my team controls the entire game from start to finish, somehow they were actually the inferior team.
Injuries:
Team structure matters. Not all backups are the same. One team might have rotations along their D line, or D line by committee, where there isn't much a dropoff from "starter" to "backup". Another team may have a superstar they lean on, and it all goes to trash when he's out. I don't know your team. You tell me what the dropoff is. Additionally, maybe your "starters" are B+ against the run and B- against the pass, but your "backups" are A+ against the run and D- against the pass. It's not as simple as "starter is out, we suck now." Well, maybe it is - again, I don't know your team. Maybe your backups are all awful, I don't know. But if all I hear is "our backups are in" then that is just as nuanced as saying 42>29. Furthermore, I don't see where you give consideration to *when* the injuries happened. I didn't see many injuries in the game. Therefore, your coaches knew who would be in on your defense and gameplanned accordingly. What would be actually catastrophic is if your coaches installed their game plan with the starters all week, and suddenly 75% of the defense is out for the game on the first drive. Now the gameplan is in the trash. Again, nuance. And lastly, the Lions put up 42 points. If you want to take away the TD on the DK fumble return, ok, 35 points, oh and we ended the game with the ball and would've had an opportunity to score even more if needed. 35 points and ending the game with the ball is still pretty good.
As a lifelong Lions fan, we had many seasons where some of us lived on "moral victory" because we had nothing else. Because we were trash. "Well we lost, but it was a good team and we looked good in such-and-such area and we can build on that." Ok. If you wanna have a moral victory, have at it. But trust me, moral victories aren't worth anything. I'm a Lions fan, I'd know.