There is, of course, the will of God as our ultimate guide. Ours is to conform to His, simply because, even by careful and painstaking analysis, especially with hindsight, His will trumps ours in all respects: efficiency, gains, timing, and so on.
There is the will of the Church, and likewise, the will of the (arch)bishop, speaking as the overseer of his diocese on behalf of the whole Church. This will is not of its own sovereign authority, but in lieu of the anointing by the Holy Spirit, it is taken to be conforming to the will of God. We can decide either way in Fr. Bob's case if the archbishop is himself conforming to and passing on God's will or not.
And then there is the will of Fr. Bob. That he would pit the will of the parish "elders" against that of the archbishop reveals a sadly rebellious spirit.
As for whether or not the archbishop (or canon law) is speaking the will of God or not, I suspect that the archbishop is being more prudent and discrete by not speaking of other reasons for Fr. Bob to retire. If only that he is beginning to confuse the Gospel and setting himself up as an idol, there is a very good reason for Fr. Bob to seek, not the will of his "elders" in the parish, but the will of God, the same God who instituted the office of bishops with his own authority and Holy Spirit.