His talents seem to lie in the admnistrative or service
We all know what happened in OTL to George McClellan. He was not the most successful general...i would say mostly because his his age and so much was thrust upon him.
....that said, what other roles would suit him. And given that he did seem to be a prodigy of sorts. He could have been an engineer in the Army, or perhaps even a Chief of Staff. Or maybe even a corps commander(most curious about this one, of him being in a subordinate position in the army)? How would McClellan fit in such roles?
His talents seem to lie in the administrative or service side, rather than as a combat commander. He was certainly not an A.A. Humphreys.
To be fair, he was successful against the rebels in West Virginia (geography helped) and at least held his own Virginia and Maryland as an army commander on the defensive (he had some able subordinates, most of who get overlooked in the McClellan go round).
The questions are a) if McDowell is to be relieved as field commander in the east in 1861, than by who?; and b) if Scott is to be relieved as general-in-chief in 1861, than by who?
Setting aside GBM as the obvious choice (for his abilities
and his prewar connections with Lincoln), personally, my choice for G-in-C would have been JKF Mansfield; with a strong G-in-C, McClellan's problems as an army commander might not have been so marked (basically, this is what I suggest in BROS).
Failing that, GBM seems best suited as a deputy to Thomas in the AG's Department, or - perhaps - if McDowell had been allowed to retain command of what became the Army of the Potomac, as a corps commander; presumably the I Corps in place of McDowell. This places him among peers that include (presumably) Sumner, Heintzelman, and Keyes.
The other possibility would be to leave him in the west and DC Buell as a division and then prospective corps commander in the east; this would put GBM in as commander of the Army of the Ohio in 1862, rather than Buell.
In which case McClellan would report to Halleck, places him among peers that include Grant, Thomas, and (potentially) Pope or Curtis.
My guess is his reputation would be much better if he had served as a corps commander after West Virginia, and before any army-level service.
Best,