What you call intangibles (which, as you said yourself, is immeasurable) in this case, is just a sizing up of the managers. I've caught hundreds of Tiger games and you're just going by the devil you know here. Leyland makes a lot more head-scratching decisions than Farrell, especially with bullpen management.
As for defense, i don't understand why you'd use dWAR over UZR, since the latter is widely regarded as the superior defensive statistic, mostly because it adds positional adjustment to fielder value and the evaluation method is better.
Using UZR, the numbers are as follows:
Red Sox: 21.6 UZR
Tigers: -12.9 UZR
Per this measurement, the Boston advantage looks even bigger, given their superiority over the central line (C, 2B, SS, CF). A word of warning regarding dWAR and UZR regarding C/2B/SS is that they are both very unreliable for C, and can be for 2B/SS.