I agree that Theon ever thinking he could take and hold the north is a major plot hole. Even holding Moat Calin, there are enough northerners still in the north that Theon could never convince a sufficient force to disobey his father, abandon their strength at sea and attempt to hold a continental land mass with a raiding party off a few ships.
However, I'm going to take the conversation even further afield because I'm rereading the books and I just don't see how Cat and Robb could be so dumb as to leave Bolton at their backs. BTW, part II of the Northern Conspiracy was just posted:
https://www.youtube.com/watch?v=3IlzXQVKpbM
It left me even more confused and more motivated to get to the last two books in my current reread so I can make sense of it.
So here's MY plot hole. Early in the war, Robb succeeds in securing two choke points of incalculable strategic significance: Moat Cailin and the Twins. If Walder Frey tells Robb to fuck off, he has no choice but to march down the King's Road and meet Tywin's massed force. With the twins secured, Robb can now make his gambit to break the siege of Riverrun and gather the river lords under his banner.
When the decision is made to split his forces and try to win through to Riverrun, Cat convinces Robb to put Bolton in command of his Eastern force. Cat's desperation to break the siege of Riverrun is understandable, but there's no question that they realize the risk they're taking. They're attempting a sneak attack on Jaime that could leave them all dead. They know that Bran with no army can't hold the North if their retreat is cut off. If their suicide mission fails, as it stands a strong chance of doing, the only hope for House Stark to avoid extinction is to be able to retreat back through the Twins to Moat Cailin.
If the Stark line fails, who is first in line to rule the North after them? Cat and Robb would know the answer to this question instantly.
Once Bolton is given the Eastern command he does exactly what you would expect him to do, given House Bolton's historic rivalry with House Stark. He rushes his forces headlong into Tywin's while keeping his personal force in reserve, thus extinguishing as much Northern strength as he possibly can while preserving his personal forces.
Sure, Robb is a boy and Cat is one of the stupidest characters in the series. But you can't tell me they didn't have some loyal commander from their personal forces at Winterfell to stop them and say "overextending all the Stark forces while leaving Roose Bolton in command of your rear is an incredibly stupid idea."
Robb should have left commanders and forces of direct loyalty to the Starks to hold Moat Cailin and the Twins. Instead of Bolton pushing the forces of the other Northern lords ahead of him like lambs to the slaughter, the opposite should have happened. Robb should have had the bulk of Bolton's forces as the vanguard from the Whispering Wood all the way to the Battle of the Camps. If they lose, the cavalry retreats to the Twins and Robb possibly wins back through to Moat Cailin with enough of his personal force intact to hold the North indefinitely, even if he has lost the Riverlands to the Lannisters.
Secondly, the orders for the Eastern force should NEVER have been to engage Tywin's force, much less rush headlong into them at the end of a forced march as they sit there well provisioned and rested. With the strategic choke points of the Twins and Moat Cailin secure, the Eastern force should have been sent to execute a feint against Tywin, harry his scouts and foragers and hope to lure as much of Tywin's force north as possible. The Twins can only be besieged if you have an army on both sides of the Green Fork. If the Eastern force succeeds in luring Tywin north, that force simply retreats until it divides to reinforce Moat Cailin and the Twins and Tywin is stuck looking at two unassailable fortifications and his forces well and truly divided.
Now, great commander that he is, maybe Tywin never takes the bait and leaves his Eastern army sitting on the Trident. This is still a positive result for Robb because if Tywin fails to besiege the Twins from the East, then his retreat to Moat Cailin is not cut off by land. The only possible bad result for Robb is what actually happens: A large portion of his Eastern force is expended because he left Bolton in command and his retreat is cut off due to Bolton's entirely predictable ambition and treachery.
Even if Tywin besieges the Twins from the east and Robb is utterly defeated by Jaime west of the Green Fork, Robb's force can still retreat to Seagard and take ship to Barrowton to reinforce Moat Cailin from the north. Sure, they might encounter difficulties with the Iron Islanders, but ships packed with battle-tested warriors are no easy meat for pirates. Whatever remains of Robb's force that can't fit onto the available ships reinforces Seagard and the Twins in a last stand.
Sure, I say all of this with the benefit of hindsight. But this strikes me as a major plot hole brought about by necessity. GRRM scripted the books out a certain way and he needed for Robb and Cat to fall right into Bolton's trap to tell the story he wanted to tell. As I said, though, there would have had to be SOMEBODY loyal to House Stark present at the Twins when the decision was made to divide forces who could have told Robb and Cat what a bad idea it was to leave Bolton in their rear. I call bullshit.