He ends up becoming Kearny?
I wonder to what extent McClellan's observation of the seige of Sebastopol obscured his thinking about big slow moving sieges and logistical concerns.
What if instead McClellan is an observer of the more fluid battles of the Franco-Austrian War?
Mansfield was USMA, second in the class of '22; served as an engineer, including as cheif engineer under Taylor in the northeastern Mexican campaign, and was brevetted major, lt. colonel, and full colonel. He saw action and led troops at Monterey and Buena Vista - so, basically, he was a well-educated, smart, engineer/combined arms officer who had been recognized three times for gallantry by the age of 45.
Not bad.
Then, in 1853, he was recommended as inspector general of the army (staff rank: full colonel, RA), in which post he served for the next years, working directly for Scott as general-in-chief and touring every post (including the arsenals and armories) and meeting a wide range of the serving active duty officers of the prewar Army.
Again, not bad, and explains Scott's high regard for him.
In May, 1861, Lincoln appointed him brigadier general (RA) and Mansfield was assigned to command the defenses of Washington; he seized and fortified the south side of the Potomac, and laid out the initial defenses of the capital.
So, all in all, he's the closest equivalent of Scott (or Lee, for that matter) in blue in '61, and he was only 57 - four years older than Lee - and in excellent health.
He would have been my Old Army pick (and in fact is, in BROS) for G-in-C, to replace Scott.
Best,