Couple of possibilities suggest themselves, in terms of
Couple of possibilities suggest themselves, in terms of the US military command.
First, Scott, despite his ailments, was the keenest strategist on the continent in 1861, and with an able chief of staff and bureau chiefs, certainly could have functioned well as general-in-chief; keep him in DC (rather than replacing him with McClellan) in 1861, and the overall war effort in the east, certainly, would go better. Give him AA Humphreys as a chief of staff, as an example, and that's an effective team - need someone else for his "unofficial" mission to France, but one could suggest Jerome N. Bonaparte would have been an interesting substitute.
Second, giving JKF Mansfield the command of the (US) Army of Northeastern Virginia at first Bull Run could have made a difference; McDowell was certainly a lot sharper than he is given credit for, but Mansfield had a lot more field experience - my bet is his plan for the campaign would have been much less complex than McDowell's, so the result would be better - whether the change would be enough to gain an actual victory, or even a stalemate on the battlefield, of course, would be an open question. Given McDowell a position as Mansfield's chief of staff would presumably be helfpul; replacing Patterson in the Shenandoah would not hurt, either - interesting choice there could have been John Wool; not any younger, but certainly more active.
Say the breaks are evenly divided, and the result is a drawn battle in both locations; given JE Johnston as the senior rebel commander, likely the rebels withdraw to the south, possibly as far south as the Rapidan. That gives the US a victory from Day One in the east (as they had in the west, thanks to Grant at Henry-Donelson) and the rebels look even weaker than they were historically.
If Scott does fade over the winter, than Mansfield is well-placed to replace him as G-in-C, with Humphreys continuing as chief of staff; EV Sumner can replaced Mansfield with the field army, with - perhaps - McDowell remaining as chief of staff at that level. By the spring of 1862, the now much enlarged US field forces drive overland, not unlike Grant's 1864 strategy, and find themselves battering their way south, river line by river line, against JE Johnston, PGT Beauregard (unless Beauregard goes west to serve with AS Johnston), GW Smith, and (presumably) RE Lee.
Henry Halleck remains a theater commander in St. Louis, and Grant is given full command of the trans-appalachian theater east of the Mississippi after Henry-Donelson; he wins Nashville and takes Memphis in the summer, and then drives east on Chattanooga (Buell takes Knoxville).
By the autumn of 1862, the US forces are besieging Richmond, poised to drive on Atlanta, and already control the Mississippi.
Lincoln gets Richmond and Atlanta as Christmas presents, and the whole rebellion is wound up by the spring of '63, two years ahead of time.
Best,