It's quite simple, it wasn't a miscalculation. They knew Mojo was probably being moved due to the asks from the 77, 9, & 92. It wasn't a miscalculation, it was the decision to accept he was a cap causality because of their value of the above 3 guys over Mojo.
I'm not sure anyone was lining up to take Mojo while the season was still in progress. You keep mentioning these ideas about how he should have been handled, but the speculative portion of the equation you're suggest is massive. You don't know what if any market there's was for Mojo, before or after the ED, you don't know the suitors if many, and you don't know the offers, but you act like moving him before was a no brainer and they should have gotten a better return. I'm just saying we don't know and these are all huge factors. Me, I'd take a 2 & a 3 from the Devils over a marginally better offer from a hated Eastern rival. I think it's more likely there were very few suitors who were willing to give up much and the Caps certainly weren't doing a stronger Eastern team any favors.