Some petty speculation: I believe, based on a break in their correspondence, that the officer he wouldn't have at Vicksburg was Franklin.
On the evening of October 15th, 1862, a few days after McClellan had placed me in command of the Second Corps, then at Harper's Ferry, the commanding general sent an order for Hancock to take his division the next morning on a reconnaissance toward Charlestown about 10 miles distant.
About 10 in the morning General McClellan reined up at my headquarters and asked me to go out with him to see what the troops were doing. Our people had met the enemy's outpost five miles from the Ferry, and while artillery shots were being exchanged, both of us dismounted, walked away by ourselves, and took seats on a ledge of rocks.
After a little while, McClellan sent to an aide for a map of Virginia. Spreading it before us, he pointed to the strategic features of the valley of the Shenandoah, and indicated the movements he intended to make, which would have the effect of compelling Lee to concentrate in the vicinity, I think, of Gordonsville or Charlottesville, where a great battle would be fought.
Continuing the conversation, he said, "But I may not have command of the army much longer. Lincoln is down on me," and taking a paper from his pocket, he gave me my first intimation of the President's famous letter. [The editors of the Century make this out to be Lincoln's letter of October 13 to GBM.]
He read it aloud very carefully, and when it was finished I told him I thought there was no ill feeling in the tone of it. He thought there was, and quickly added, "Yes Couch, I expect to be relieved from the Army of the Potomac, and to have a command in the West; and I am going to take three or four with me," calling off by their names four prominent officers. I queried if "so and so" would be taken along, naming one who was generally thought to be a great favorite with McClellan. His curt reply was "No, I sha'n't have him."
This brief conversation opened a new world for me. I had never before been to any extent his confidant, and I pondered whether on a change of the commanders of the Army of the Potomac the War Department would allow him to choose the generals whose names had been mentioned. I wondered what would be the future of himself and those who followed his fortune in that untried field.
These and a crowd of other kindred thoughts quite oppressed me for several days. But as time wore on, and preparations for the invasion of Virginia were allowed to go on without hindrance from Washington, I naturally and gladly inferred that McClellan's fears of hostile working against him were groundless. However, the blow came soon enough. [It came immediately after mid-term elections.]
How do we interpret Couch's comment about those officers "the War Department would allow"? There seem to be three possibilities: (1) the officers McClellan wanted for the Mississippi were all candidates to replace him in the east (2) They were all officers his replacement would want to keep (3) They were beyond his reach politically, that is in his Eastern relief he would have no Western choices.
Now on to the major speculation. What might we think McClellan's evaluation of the situation was based on his interactions with Wilson and Couch?
(1) He felt he received a "solid" offer of command in the west from an Administration go-between. He knew that Lincoln does nothing without cut-outs and deniability.
(2)He felt he did not refuse the offer. He told Wilson/McClernand that he would not willingly give up the AOP but that he would "think about" serving in the west.
(3) He told both Wilson and Couch that he would be relieved. He believed that he would be relieved.
(4) His subtle, conditional accpetance after Wilson's speech he erroneously believed to have registered. Instead, Wilson registered it as an unequivocal "no."
(5) There is nothing to suggest in his correspondence during his long stand-by in Trenton that McClellan was seeking to return to duty on his own terms. Quite the contrary.
McClellan was prepared to command the Vicksburg campaign. Wilson misread McClellan and killed that possibility. McClellan was for Vicksburg.