What were the Marines up to? [backstory]

[Legbreaker]

The real problem I see (which has been discussed at length in other threads) is a lack of understanding of the overall picture. There's no way the 8th could possibly have moved so far east without specific orders to that end. Given that, and the knowledge the Marines made amphibious landings across northern Poland, the actions of the component units makes sense.
Essentially the 8th were the spearhead and crossed the estuaries of northern Poland across the "carpet" laid by the landings of the Marines.
The 5th ID had flank protection, and the rest of the XI Corps were to follow after those three units.

Behind XI Corps you have the rest of the III German Army who were supposed to push up behind XI Corps, and possibly leapfrog them at some point. Meanwhile the British and other units left in the Germany/Poland frontier were to shift around to cover the gaps the assaulting units left.

Unfortunately, only XI Corps had made any headway before the Pact launched their own summer offensive and with the two sides essentially bashing into each other head on, the result was almost inevitable.

It's likely that if one side had chosen to stay on the defensive, and had managed to hold the other, the war could well have dragged on for at least another year (probably two since it took them both that long to prepare for the 2000 offensives). As time dragged on though each side would have become progressively weaker due to wearing out machines and consumed ammunition while the stalemate was maintained. Eventually the front would have become so porous that it couldn't even be called a front anymore.

The end was virtually inevitable, the timing was really the only question.