I agree with you on the long-term win for the Patriots, but your first point about Mankins on the field against the Dolphins? Nope.
The Pats scored the very same amount of points this year in Miami as they did last year and the TOP was right around the same. And last year they had a far better, more punishing RB group with Blount instead of James White.
Same points: the Pats scored all their points in the first half this year, going 4 for 6 on drives. Last year was the game after losing Gronk and the Pats were naturally ineffective in the red zone, going 1 for 4. This year, the Pats had Gronk, he scored a TD, they went 2 for 2 in the red zone, the problem was they couldn't move the ball in the second half. They had something like 50 yards of offense after halftime, and went 0 for 6 on drives.
TOP: The Dolphins possessed it less this year because they were given short fields due to offensive turnovers caused by the pass rush. The Pats ran 37 plays in the second half (vs. 43 in the first half), but moved less than 50 yards, with 15 plays and 37 yards coming on their final drive against a prevent defense.
Against the Dolphins last year, the Pats allowed one sack. This year, they allowed four sacks. Yards per pass was down from 6.5 to 3.8! 5 is mediocre.
Excepting the final drive, the Pats ran 18 pass plays for 10 yards of offense in the 2nd half. The last drive was 13 plays for 25 yards. Together, that's 1.3 yards per pass. In the first half, the Pats averaged 7.4 yards per pass.
I didn't have Devey down for many sacks, those came from Brady's blind spots outside the tackle box, but he had by far the most pressures allowed.
I think Mankins, or anyone really, would have resulted in a vast improvement over Devey.
EDIT: also I think Solder left the game last year with a concussion so the Pats had to kick out Mankins at LT to go with Cannon and put in Kline. With backup tackles and a backup guard to go along with Wendell at center the Pats still moved the ball and came within a play of winning, they just had no red zone offense. This year, the Pats were one warm body (Mankins, Kline) away from moving the ball at all.
Don't see these games being very comparable, only contrastable.