CF Smith's assault at Donelson was at a fortified position which was suspected of being undermanned.
Yes, and that turned out to be correct. We know the manning levels of the Yorktown line.
Missionary Ridge should have been a very strong defensive position for the confederates.
But they hadn't dug in; at Yorktown the line was dug in, and there were defiles (the dams), and it was screened by a flooded river.
McClellan outnumbered the confederate defenders on the Yorktown line
But not by much - not enough to make an assault easy.
Can you give an example of a dug-in army which was broken through by assault at similar force ratios to the Yorktown line? Indeed, I'd say that if McClellan could have broken into the Yorktown line then Grant should have been able to break into Vicksburg, because the force densities are similar if not greater at Yorktown.
Look at previous threads for the links and references to the firsthand accounts of people who claimed an assault could have succeeded. I'm not going to look that all up again.
Can't you at least search your own material? You're more likely to know the terms to use.
There's a mention of WF Smith having thought he could penetrate the line and that Hancock was involved, but it's not in the ORs - it isn't mentioned until 1881 (by Smith) and Hancock denied it when asked to confirm. This is very dubious as it's two decades after the event and of the two people involved one of them said it didn't happen!
Hancock's report does exist in the ORs, but it's actually two reports mixed together (one of his recce on the 6th where he couldn't approach the Garrow Ridge and one of his recce on the 9th when he could).
Ah, found your post where you quote people. You quoted:
Joe Johnston (confederate):
"No one but McClellan could have hesitated to attack."
John Magruder (confederate):
"to my utter surprise he permitted day after day to elapse without an assault."
E.D. Keyes (union):
"My impression now is, that, if the whole army had been pressed forward, we should have found a point to break through."
S. Heintzelman (union):
"I think, if I had been permitted when I first landed on the Peninsula to advance, I could have isolated the troops in Yorktown, and the place would have fallen in a few days."
A.S. Webb (union):
"Thus a fair opportunity to break the Warwick line was missed. Had the same effort been made when the army first reached the line, there can be little doubt that success would have attended it."
E.P. Alexander (confederate):
"Another opportunity as good as that offered McDowell at Bull Run was here offered to McClellan, who could have rushed the position anywhere. He contented himself, however, with some cannonading and sharp-shooting."
To take these Confederate first:
Johnston's information is based on Magruder's reports.
Magruder was claiming McClellan's army was between 100,000 and 200,000, and that he himself had only 5,000 to cover the full length of the Warwick; this is false.
E.P.Alexander arrived with Johnston, and the quote you give is from 1907 in a book in which he claims that the force in Yorktown on April 5th was only 13,000 and that the length of the line is 12 miles; this is also false. (On the 5th he had Rains, Crump and a few pickets
not on the line, which gave him about 13,000 infantry exclusive of Ward and Rains and also about 1,600 artillery effectives exclusive of them, though of these 720 are the heavy artillery reserve and may also be in Yorktown) The Yorktown line is also self-evidently not 12 miles long.
Since all the Confederates are working from false information on this matter, we can disregard them.
Keyes' quote is later on - at the time he said that fifteen divisions wouldn't be enough and when ordered to attack the line "if only with the bayonet" on the 5th he refused.
Heintzelman's quote is saying that he thought he could have taken Yorktown in
late March (which is when he first landed on the Peninsula, with two divisions). He like Keyes was surprised by the strength of the defences.
And A.S.Webb is talking about Garrows Chimney, which wasn't discovered by anyone until the 9th of April; it's thus hindsight.
Nobody on the Union side claimed at the time that the line could easily be breached.
I should also note by the way that you didn't actually source any of those, and I couldn't verify the Keyes quote with Google. This conceals for example that the EP Alexander quote is from a book from 1907.
Here's my own much longer Keyes quote:
MY DEAR SENATOR: The plan of campaign on this line was made with the distinct understanding that four army corps should be employed, and that the Navy should co-operate in the taking of Yorktown, and also (as I understood it) support us on our left by moving gunboats up James River.
To-day I have learned that the First Corps, which by the President's order was to embrace four divisions and one division (Blenker's) of the Second Corps, have been withdrawn altogether from this line of operations and from the Army of the Potomac. At the same time, as I am informed, the Navy has not the means to attack Yorktown, and is afraid to send gunboats up James River for fear of the Merrimac.
The above plan of campaign was adopted unanimously by Major-General McDowell and Brigadier-Generals Sumner, Heintzelman, and Keyes, and was concurred in by Major-General McClellan, who first proposed Urbana as our base.
This army being reduced by 45,000 troops, some of them among the best in the service, and without the support of the Navy, the plan to which we are reduced bears scarcely any resemblance to the one I voted for.
(Keyes, writing to Ira Harris on the 6th or 7th April. Note that he was one of the ones who voted for the plan, as he states)
But he was able to work well with his superiors and with peers, such as the Navy, so that much of the drama and misunderstandings of '62 may not have happened under Grant.
Well, if the local navy commander on the York was willing to fulfil the Navy's promise of support, then yes there would have been an easy way to get through Yorktown. But that's not really dependent on McClellan vice Grant as such.