Hypothetically, if the Red Wedding had been bungled and blown up in Bolton’s and Frey’s faces -the plot is discovered early or the northmen somehow mounted an effective resistance- what would the ramifications have been?

Depends on how early it’s caught. If it’s caught before Duskendale and Ruby Ford, for example, the Northern army is up 5,500 men and some of their best battlefield commanders, Robb retakes the North. 

Main difference would seem to be what happens to the Freys – do they go back into pocket? Do they start killing eachother? – and the Boltons, who won’t be ruling the North any time soon.