At Antietam the only forces McClellan does not commit to the battleline are Barnes' brigade, Morell's Division, 5th Corps, and 4 regular battalions of Sykes' division (3rd US, 2/12th US, 1st/6th US (consolidated) and 1/17th US). As a proportion of available forces it's more than Lee committed over 3 days at Gettysburg (or indeed Meade).
I apologize for focusing on this rather than your other points, but I want to address this because I think what you have to say is severely at variance to the description given by everyone else to the point that I don't think its credible.
Unfortunately we don't have a report for Porter, but we do have the following:
http://antietam.aotw.org/exhibit.php?exhibit_id=55 Sykes
"On the 17th some light batteries and cavalry, under General Pleasonton, were thrown across the Antietam via the turnpike bridge. Their position being one of great exposure, and General Pleasonton desiring some infantry to protect his guns, I sent the battalion of the Second and Tenth U.S. Infantry, under Capt. J. S. Poland, Second Infantry, to his support. Subsequently, the ammunition of Pleasonton's batteries having been exhausted, I was ordered to relieve his guns by my own, and accordingly (against my judgment) sent Randol's and Van Reed's batteries across the Antietam, and with them four additional battalions of regular infantry, under the command of Capt. H. Dryer, Fourth Infantry."
http://antietam.aotw.org/exhibit.php?exhibit_id=158 Dryer: "We remained on this ground until about 2 o'clock on Wednesday, when General Sykes gave me an order to cross the Antietam with the Fourth and First Battalion of the Fourteenth, and to take command of all the regular infantry of his division on that side of the creek, consisting in all of the Second and Tenth, commanded by Lieutenant Poland; Fourth, and First Battalion Twelfth, Captain Blunt; First Battalion Fourteenth, Captain Brown; and Second Battalion Fourteenth Captain McKibbin"
So at most five battalions/regiments/collections of companies out of twelve in the division, not counting Warren's two regiments, which is not even
mentioned in Sykes's report (sadly no report of his own), let alone described as doing anything outside of reserve.
And you say that McClellan has committed everything from that divisiion except four regular units?
Based on
what inquiring minds want to know?
Griffin reports his brigade's (and Stockton's) actions as follows:
"
On the 17th the brigade was not in action. About 4 p. m. an order was received from Major-General Porter to move to the support of General Franklin's command. The brigade moved about half a mile to the right, where it was halted, together with the Third Brigade, same division, by General McClellan, with directions to remain there until further orders, and returned to the position occupied in the morning, just before sunset, by direction of the general commanding, and encamped for the night."
I suppose you could technically say that those two brigades weren't just left in reserve to twiddle their thumbs, but "moved half a mile to the right and then halted without any form of action" . . .
I suppose that's better than forgetting they existed and leaving them completely idle, but to describe them as "committed to the battleline" is well beyond what is described in Griffin's report.
McClellan's own report -
http://antietam.aotw.org/exhibit.php?exhibit_id=153 - says the following on 5th Corps:
"
General Porter's corps, consisting of General Sykes' division of Regulars and Volunteers and General Morell's division of Volunteers, occupied a position on the east side of Antietam Creek, upon the main turnpike leading to Sharpsburg, and directly opposite the center of the enemy's line. This corps filled the interval between the right wing and General Burnside's command, and guarded the main approach from the enemy's position to our trains of supply. It was necessary to watch this part of our line with the utmost vigilance, lest the enemy should take advantage of the first exhibition of weakness here to push upon us a vigorous assault for the purpose of piercing our center and turning our rear, as well as to capture or destroy our supply trains. Once having penetrated this line, the enemy's passage to our rear could have met with but feeble resistance, as there were no reserves to re enforce or close up the gap.
Toward the middle of the afternoon, proceeding to the right, I found that Sumner's, Hooker's, and Mansfield's corps had met with serious losses. Several general officers had been carried from the field severely wounded, and the aspect of affairs was anything but promising. At the risk of greatly exposing our center, I ordered two brigades from Porter's corps, the only available troops, to re-enforce the right. Six battalions of Sykes' Regulars had been thrown forward across the Antietam Bridge on the main road to attack and drive back the enemy's sharpshooters, who were annoying Pleasonton's horse batteries in advance of the bridge. Warren's brigade, of Porter's corps, was detached to hold a position on Burnside's right and rear, so that Porter was left at one time with only a portion of Sykes' division and one small brigade of Morell's division (but little over 3,000 men) to hold his important position.
. . .
General Sykes' division had been in position since the 15th, exposed to the enemy's artillery and sharpshooters. General Morell had come up on the 16th, and relieved General Richardson, on the right of General Sykes. Continually under the vigilant watch of the enemy, this corps guarded a vital point.
The position of the batteries under General Pleasonton being one of great exposure, the battalion of the Second and Tenth U.S. Infantry, under Captain Poland, Second Infantry, was sent to his support. Subsequently four battalions of regular infantry, under Captain Dryer, Fourth Infantry, were sent across to assist in driving off the sharpshooters of the enemy.
The battalion of the Second and Tenth Infantry, advancing far beyond the batteries, compelled the cannoneers of a battery of the enemy to abandon their guns. Few in numbers and unsupported they were unable to bring them off. The heavy loss of this small body of men attests their gallantry."
Yes, it might involve some element of risk to use 5th and 6th Corps aggressively to smash Lee in the hopes of rendering his army unable to launch a vigorous assault "for the purpose of piercing our center and turning our rear, as well as to capture or destroy our supply trains.", but set against that is that if things go well it might even pierce
Lee's center and turn
his rear.
McClellan is not in the 18th century fighting a limited war with a too-expensive-to-readily-replace army or fighting in a situation where his army remaining intact but inflicting heavy casualties is more of a victory than capturing ground and claiming victory on the idea that he left the battlefield after his opponent.
In one of those situations, it would make sense for him to be concerned with Sumner advising caution, even if the enemy might also be heavily battered. There are others, but those are more relevant than talking Byzantine history in the 9th century.
In
this situation, it would be far better service to the national cause for Lee's center to be pierced and his rear threatened at a bloody price than for McClellan to opt out of the risks and costs of using 5th and 6th Corps in the hope of doing so.