I don't think whose team makes the playoffs should be part of the equation. Its defensive player of the year. Its not 'who played both sides of the ball and got their team to the playoffs'. These guys can't control what the offenses do. So they shouldn't be held accountable for their team records.
I think you can look at where their teams rank in points allowed. Lets keep it to Merriman and Taylor. The Dolphins are 5th in points allowed. The Chargers are 8th in points allowed.
JT has 12.5 sacks, 2 INTs, 10 passes defended, 2 TDs, and 9 forced fumbles.
Merriman has 12.5 sacks, 1 INT, 5 passes defended, 0 TD, and 4 forced fumbles.
Looks like JT has the edge in every category but sacks, where they are tied. And the Dolphins have given up fewer points, even while their offense is ranked 26th in INTs thrown while the Chargers are 2nd in INTs thrown. So even while being put in worse positions turnover wise, they have performed better.
I don't think Merriman's suspension should have anything to do with it. It should neither disqualify him for the award nor should it help him. You can't say something like, 'Well he played in 4 fewer games and therefore his numbers are more impressive'. First off, he played in fewer games because he got caught breaking the rules. That is his fault. Second, just because he missed those games doesn't mean he would have been putting up great numbers in the ones he missed. You can't just assume something like that.
So any way I can find to look at it, JT should be the DPOY.