Do you think Robb would have been amiable to relinquishing the Riverlands to the IT in return for legal recognition of the North’s independence?

He might have been willing, if he could have been assured that the Riverlands would not be treated poorly, which depends greatly on who the king was. If it’s Stannis or Renly, he might go along with it as long as there are amnesties and the like to ensure his former bannermen and current kinsmen are not mistreated. If it’s the Lannisters, unless it’s literally a matter of life and death, I don’t think he would go along with it.