The thing about combining Rothman and Garrett is where Garrett lives. A "fair fight" district in Bergen is totally feasible, but a district that includes a large portion of Bergen and stretches all of the way out to Sussex is almost impossible due to population constraints and very unlikely to be a fair fight. I suppose they could put Garrett and Frelinghuysen (or Garrett and Lance) together and then make Rothman's district marginal to "compensate" (unfair since Republicans are already overrepresented, and Lance's district is obviously the one that "should" be eliminated, in addition to Pallone being drawn out of his district [but the district itself surviving as a D seat], but whatever), but that doesn't seem like what the commission is suggesting.
Republicans have nearly as many seats as the Democrats because the voters of New Jersey chose to elect them. It simply isn't for you to decide which parties are "overrepresented," and which are not.