But... I really think it would take multiple PODs to make the South win without British help. OK, let's say McClellan falls for the trap (as after the war General Sherman insisted he should have done). OK, we lose the entire army of the Potomac, or maybe the cavalry just abandons the army, splits into groups of 30, and flees (Lee can't catch them all). OK, so we're going to need a few draft slips and it will probably take 3 years to rebuild it to full strength. So the Civil War ends in 1868.
Well, think about the operational situation. Firstly, there's nowhere for the cavalry to go except Fort Monroe (which is many miles away).
With the Army of the Potomac gone, essentially the entire Army of Northern Virginia (which is actually slightly larger than the AotP at this time in effectives terms) is free to head north and attack against Washington.
Now, you may be aware of the abject defeat that was Second Bull Run? Imagine it
without the 3rd, 5th, 6th and 9th corps of the Army of the Potomac present -
and without the rest of the AotP rapidly returning to Washington for rearguard action. The Union finds itself having to rush reinforcements in to Washington as fast as possible, needing to replace about 25% of their entire army (and that mostly the experienced part) and having to pretty much send nearly
half the force of the Department of the Mississippi eastwards in order to make up the numbers - which in turn means the Western Theatre is unable to make an effective offensive.
It's nearly as bad in terms of non-available manpower as a war with Britain, for the Union.
At the same time, Lee's army has just acquired a large quantity of reasonably good rifles and a large number of good artillery pieces, exactly what would be needed to actually make an attempt at the Washington defences.
It's not valid at all to just assume the Confederacy will wait for the Union to rebuild their field army - the CSA has no reason at all to wait, especially as it has just assembled the largest army it would ever field in OTL.