I think that retaining Philbin and firing Ireland and Sherman is exactly the correct decision.
As to what GM would accept a job without being able to hire his own coach, I think that prospective GM's would be salivating at such an opportunity. Just look at Gettleman in Carolina who neither hired the coach nor drafted the QB.
In such a situation the new GM gets a free pass for the first year because he wasn't allowed to bring in "his guys". If the team does poorly then he gets to hire his own coach and draft his own QB. It buys such a GM a lot of time before his own performance is evaluated.
But I'm sure if Ross takes this approach he'll be labeled as the biggest moron in the world. In truth, it's the shrewdest decision he could make and the quick turnaround of the Carolina Panthers is evidence that adopting this model can lead to success. The parallels between the Panthers and Dolphins are even more relevant considering that Tannehill will also be entering his 3rd year as starter. Give him a new OC and QB coach and combine that with the natural improvements that QB's typically make between years 2 and 3...