My opinion remains that the separation of the AoP was the root cause of the failure of the Peninsula Campaign. Neither the North Wing or the South wing had power enough to accomplish much.
Well, there is simply no way to avoid separating the Army of the Potomac across the Chickahominy if the supply route is White House Landing. Incidentally, in late June the south wing (23 brigades) was strong enough to push towards Richmond now the weather was clear enough to move artillery.
There was opportunity between the 20th , when his forces crossed the Chickahominy, until the 31st when Johnson attacks to fully clear the heights and re-connnect the two wings of his forces, but it was not done. Ordering the required bridges after the enemy attacked it not useful and clearly indicative of poor command. All of this was a command responsibility and McClellan was in command.
But holding the heights over New Bridge wouldn't solve that problem! There'd still be at least one corps north of the river, and having New Bridge available to move troops wouldn't mean there were suddenly enough troops to cover Tolopatamoy Creek - that mile or two of controlled river doesn't free up the four required brigades.
It would mean it was possible to transfer reinforcements more easily, true, but that doesn't cover Beaver Dam Creek and Tolopatamoy Creek with the same force.
As far as I can tell by looking at the maps, holding the line from Seven Pines to the New Bridge Heights with sufficient density to make it the main line of resistance would mean committing at least half of Sumner's Corps if not the whole thing to an advanced line. This is frankly pretty risky, as it would mean McClellan had a small (or no) reserve available - if Casey gets hit like he was in the historical Seven Pines and collapses, then suddenly McClellan's been flanked - and it also means that there are no reserves available in the case of an attack
north of the Chickahominy. (McClellan's advancing against an enemy of roughly equal size with the benefit of fortification, and 20% of his army is elsewhere. Caution is almost required in this situation because if his enemy flubs it they can retreat into the fortifications, but if
he flubs it then he's not got any fortifications to retreat into and he's stuffed.)
It is quite possible that this risk would have been borne out, of course... but even if it had, the essential difference is that McClellan's attacks against Richmond are in the position on the 25th June (when the rains clear and offensives can resume) they historically were on the 27th. It doesn't solve the problem of how to defend against Jackson coming down on the flank,
unless (and this is impossible to tell) it means Jackson is recalled to Richmond. If Jackson is recalled to Richmond by this operation then Richmond falls in July because McClellan gets McDowell, but that's a third-order development hinging on the Confederate commander making an unforced error.
Blame the engineers if that makes you happy. I don't, they could not overcome terrain, mother nature and the overwhelming requirements placed upon them. Many of these requirement were the direct result of McClellan's decision to split the force.
But McClellan has no choice but to split the force. The Chickahominy has to be crossed, there's no way to avoid that.
Ordering the required bridges after the enemy attacked it not useful and clearly indicative of poor command.
But, um... is it not a plausible sequence of events that McClellan has previously ordered the Chickahominy bridged and is reiterating the order to get the bridges in place as fast as possible now the need is urgent? I mean, they were clearly already working on the bridges, so it's not like he asked them to bridge the Chickahominy from scratch during a battle.
This question is impossible to answer without considering the opposing force involved. The ANV in 1862 is not the same number or quality of 64-65 ANV. It's apples and oranges once you get past the commander.
Yes, the ANV of 1862 is substantially bigger than the ANV of 1864. Quality differs, it's true, but the same is true of the AotP.
But it's possible to calculate the amount of troops surplus beyond a strict defensive in the south (because all the commanders were asked how many troops they could spare - and McClellan then assumed they were pessimistic and took a bit more) and the answer is about six brigades. Those six brigades are enough to conduct a bite-and-hold if they're available in the south, but they're also about what it would take to cover Tolopatamoy Creek (the one natural line McClellan didn't have the troops to cover). Thus McClellan has enough troops to attack Richmond
or cover Tolopatamoy Creek.
Essentially making the comparison with 1864 to 1862, the assertion that comes out is that in 1862 the Army of the Potomac was at least twice as good on a man to man basis relative to the Army of Northern Virginia than it was in 1864. (1862 the campaign strengths of the two sides were even, 1864 the AoNV was outnumbered about 2:1 in campaign strength. By Lanchester Square this would imply that the 1864 ANV would have to be as much as four times as good man-to-man to make those relative force balances proportionate.)
Of course, we can rely on contemporary opinion?
In 1862 everyone agreed McClellan didn't have enough troops, they just never got around to actually reinforcing him. (June 1862 everyone agrees McClellan needs 1st Corps, July 1862 Lincoln agrees McClellan needs as many reinforcements as possible.) The contemporary argument about not reinforcing McClellan was generally that there were not enough troops
available, and for that an 1864 comparison is absolutely appropriate.