The "reserve" available to Porter was roughly 2,500 effectives strong, and consisted of ca. 1,000 unengaged regulars and Barnes' brigade of Morell's division.
There were never any "breeches" to exploit.
You're confusing Antietam with Glendale.
At Glendale what happened was thus: At 1600 hrs there was no major fighting other than an artillery duel between McCall's artillery and Longstreet's artillery which McCall was winning. McClellan, at his command post on Malvern Hill, received a note from Rodgers insisting that the army had to withdraw all the way to Dancing Point (i.e.
30 miles SE of the armies position). This was unacceptable to McClellan, who was starting to land supplies, and considered his movement complete. McClellan rode down to Haxall's Landing and talked to Rodgers. At 1645, McClellan and Rodgers then boarded the Galena and went to the Captain's cabin, leaving the Comte De Paris on deck to summon McClellan if anything happened on land. Almost immediately McClellan boarded Wise's river column was spotted and the ship went into action shelling them. In the first break in the fire McClellan sent a signal to Malvern asking what was happening further north, and got back that there was an attack on McCall...
The attack on McCall was an accident. Losing the artillery duel, Longstreet told Micah Jenkins to advance the Palmetto Sharpshooters to snipe the guncrews. Instead of doing this, Jenkins (who'd been raised to brigade commander vice RH Anderson, who was commanding Longstreet's division vice Longstreet, who was commanding a wing) charged the brigade at ca. 1630-45, and were slaughtered. Seeing this, Longstreet tried to support brigade, but the orders only went out at 1700, and this was a surprise - the brigades had gone into routine and were cooking. Hence no supporting brigade attacked until after 1730, when Jenkins' men were already repulsed.
With the news of an attack on McCall, McClellan sent out a series of orders. Amongst them were for Couch's division to reinforce the attack point. Unfortunately for posterity, the signallers noted that the network became overloaded, and they could deal with the volume of traffic. So busy were they that they didn't have time to write the messages in their logbooks, and so we don't know the full contents of what was sent (see their report in SOR 2). That done he cross-decked to USS Jacob Bell and steamed back for Haxall's. There he goes ashore and rode hard back to his CP at ca. 1800.
After the battle, instead of reporting in as battle procedure, Franklin and Baldy Smith retreated without orders. Making matter's worse,
they retreated eastwards, on the Charles City Road. Lt Newhall was
sent by McClellan to find Franklin, and redirect his errant command to Malvern Hill. Because Jackson was already across the White Oak, the Federal position was now unzipped, and there was a scramble to quickly pull together a defensive position at Malvern Hill. Malvern Hill was not defensible long term (because the rebels could now cut off supply ships) and a compromise position of Harrison's Landing was agreed upon - that being as far upriver as the Navy could supply the army.
No. He made his CP with 5th Corps HQ. He had a good view of the field (except Burnside's front) and good communications. At least once he rode to Sumner on the right, and it was his personal observation of Sedgwick's broken division that convinced him further offensive action without moving reinforcements there was a mistake.