Most of the answers to your question are going to be speculative… so here are my thoughts, and they're nothing more than thoughts.
1. Nobody truly knows
As everyone has mentioned, accountability is not transparent, so at best we are entering the wild speculation zone.
2. Group decisions
At the end of the day, it was clearly a mix of Grier, Tannenbaum and Gase. My guess is that Tannenbaum had a large say on everything, but was most involved in the free agency end of things, including closing the Gase and Suh deals. While he succeeded in those two, the Suh deal was for absurd money, and hurt the team. Then Tannenbaum did what he was guilty of doing before with the Jets--kicking the salary cap can down the road and creating a monster. I give Tannenbaum full blame for that one. Grier on the other hand was also very involved in scouting and acquiring free agents, but spent the largest amount of time evaluating college prospects for the draft--that is his primary area of expertise. Ultimately, the roster was under Grier's control, but with Tannenbaum having Ross' ear, Grier had to get Tannenbaum on board or most decisions would ultimately fail. Gase was definitely the least involved in this process, but with a strong vote, so think of him as the swing vote on everything.
3. Ross has better information than we do
One thing that Ross has shown is that he follows a typical real estate business model. That is going to require people doing their own individual analysis, and Ross is going to know who was banging the table for each recruit, be it a FA or draftee. As time goes on, he will ask his outside consultants to be subject matter experts and evaluate those players performance and fit in Miami, and grade each one. Then he is going to analyze who was banging the table for each of them and determine whose eye for talent is right more often vs. whose is wrong. While we do not know the results of this analysis, we can rest assured that Ross does. While he doesn't know football particularly well, I'm confident that he can get his experts to judge the final product and compare that to his internal notes of who lobbied for each of those guys. That makes it pretty easy to keep score on who is right. Because the only person who still has his job is Grier, and he gained power in the exchange, I think it is safe to assume that Grier had the best record scouting talent.
4. Timing suggests Grier drafts well
Looking at the drafts prior to Grier being named GM and after being named GM, the drafts with Grier as GM are better overall. Whatever influence Grier had when he was promoted seems to have bettered our ability to acquire talent. That bodes well for Grier.
So this is just my speculation, and it has no real information to evaluate, but the scraps of data we have seem to infer that Grier is a better talent evaluator than Tannenbaum … no surprise there. I discount Gase's input because coaches are too busy coaching to become good at talent evaluation. I think Gase was fired because he didn't win enough games, not because of his ability or lack of ability identifying talent. But, it's clear that talent acquisition, talent health, and talent salary cap consequences did hinder Gase's ability to win games. It wasn't everything, nor likely the biggest thing, but it definitely was a factor, IMHO.
