Let me first say I'm not defending Ireland. I'm just not convinced by the methodology you list.
I'm suggesting that to get a clearer picture of Ireland's performance, I would look more at the total value of a roster at point A vs. point B, and then compare that delta vs. that of other teams in the league over the same time period, or historically. This opposed to individual additons/subtractions of players grouped as + or -. The total value of a roster is what directly translates into wins/losses now and in the near future. The two main beefs I have with your methods are:
a) it ends up distorting the value of bit players ( see the numerous -1's listed for backups and special teams players --- I think you are understating the drag this has on the final score )
b) it can distort the matchup of player-vs-player: is Brandon Marshall truly being replaced by Legedu Nannee on our team? At one point yes, but at this point I'd argue not...
It seems like nitpicking, but it's a fundamentally different way of looking at it, and we could end up with very different numbers. For instance, after you added up the 'wins' contributed by each player, really Brandon Marshall would effectively end up compared not with Legedu Nannee but with the team's current most productive receiver ( brian hartline ). So instead of -3 you might get -1.5 The various special teams players like the 4 linebackers mentioned, instead of a -4 could total to be a -1 combined ( since most of them barely play ). Tannehill as a starting QB matters way more to the way more to the overall roster value than everybody else. How do you weight this? My guess is if you believe Tannehill is materially more valuable towards current and future wins than Henne, this value would probably be more like a +5. Cam Wake, by having improved, may provide a +3 or something. Jake Long by regressing a -1. And so on....
You might think however, you'd also need to factor in the fact that every year teams get 7 draft choices, so that x3 years is no small infusion of value. However this is offset by players aging and decline.
Again my point is not to defend Ireland but to come up with a potentially better approach....I have no idea how the final values would shake out from this. Something like this though is needed before we can begin to give a valuable tally.