So I was thinking about what the outcome could have been if McClellan had had 6th Corps cross the Potomac and try and block it behind Lee, and to my mind the key issue is what Lee does in reaction.
Basically Lee has a resource allocation problem here. 6th Corps had about 9,000 combatants in the divisions of Smith and Slocum, so about 14,000 effectives total with Couch is a reasonable estimate
after the marches.
Lee has several choices. One of them is for AP Hill to just basically hold the possible crossing point/s of the Potomac and block Franklin (which he should be able to do, AP Hill's division was one of the large ones and a river crossing like that is easy to hold) though this would rob him of AP Hill during the battle itself. This might come out a wash or even detrimental for McClellan.
If Lee had already ordered AP Hill to join him and Franklin gets over the Potomac (I'm not sure how plausible this is,
@67th Tigers ?) then Lee's options shrink. He can have AP Hill act as a rearguard by himself (which would do the same in terms of the impact on Lee's force as the previous option, but risk defeat much more for AP Hill - in campaign strength he'd be outnumbered about 2.5:1 and in effectives it's about the same) or add McLaws or Anderson to the rearguard, which would make it able to fight pretty much an even battle with Franklin in return for not having those divisions with him on the 17th.
Leaving both McLaws and Anderson to fight, along with AP Hill, means the serious risk of Franklin' defeat, but it also means that the fight at Antietam on the 17th is a likely Union victory.
And having Jackson's entire force from Harpers Ferry fight Franklin means Franklin's toast - but it also means the almost certain destruction of Longstreet's wing on the 17th.
My suspicion is that in fact crossing the Potomac there was not practicable after the Rebels left, but it's interesting seeing the risk/reward of what would happen if Franklin
did cross. Functionally either Lee commits enough force south of the Potomac to fend off Franklin that the main fighting probably comes out a Union victory and Lee has to retreat to Boteler's Ford in disorder, or he doesn't commit enough force south of the Potomac and so Franklin ends up covering the southern side of Boteler's Ford in force - and Lee's army is screwed, as it's stuck unable to break out and surrounded by rivers.
It'd be an interesting alternate history scenario.