Answer to your question: Michael Boulware
Boulware had that side of the field and he let Jackson slip behind him. This is very interesting b/c Boulware had been benched for the previous 8 games for letting a guy get behind him in the Minnesota game. But with the Seahawks (especially his replacement Jordan Babineaux) struggling to stop the run over that span, Holmgren put Boulware back in the starting lineup today b/c he is a stronger tackler (remember, he played LB in college). So it was a cruel twist of fate that saw the Seahawks lose that game today, but by no means was it a coincidence.
2) I understand where you're coming from, I call it winner's remorse. It seems like things just fall into place and your squad gets lucky here and there. I remember last year when the Seahawks, on their way to the Super Bowl on a similar late season hot streak, beat the Giants b/c Jay Feely missed 3 game winning kicks. It's a bunch of bullshit, don't worry about it. In the parity-dominated NFL, only special teams go 13-2. The Chargers have earned everything they've gotten. It doesn't guarantee playoff success, but don't dwell on the breaks b/c good teams make their own breaks, and that's what the Bolts have done.