You’re right about the interior not being completely landlocked, but the Mississippi isn’t as navigable as you might think. The vast majority of cargo ships (pretty much anything designed for the open ocean) isn’t going to make it further north than Baton Rogue because the river is far too shallow. Baton Rogue is a fine city (New Orleans too), but they simply don’t have the infrastructure to support multiple states in the middle of a war, not to mention the blockade that WOULD exist. This was even a problem both sides had to deal with during the Civil War; it’s not a new development, and its not something that can be fixed without an enormous amount of money and manpower. Plus there are LOADS of hairpin turns, narrow passageways, and other complications that make operating multiple barges in any one location a nautical traffic jam exceeded only by the Evergreen/Suez fiasco.
The Great Lakes have some of the same issues, but like others have already pointed out, it’s highly unlikely that Canada would just sit back and let multiple feuding parties pass by their major population centers without SOME form of intervention. Idk, they’re definitely a wild card, but this argument is entirely dependent on whose side Michigan (specifically Detroit) would choose. If they choose to support the ‘exterior states’ then there goes your best northern interior port just like that.
Lastly, all of this is ignoring that we have aircraft and carriers now, rendering most of these considerations pointless. Even if the military itself chooses to stay out of it for whatever reason (unlikely), the existence of airpower is sure to make any future American Civil War 2 a lot more complicated, even if it’s just random dudes in Cessnas dropping pipe bombs or whatever else
“The hero we need, but not one that we deserve”