It is just my opinion, naturally, but I think the NEC created the split by reinstating Corbyn. At that point Starmer had a choice of exactly what kind of split, but a split- or at least a major row - was unavoidable.

For my money, the best solution would have been to draft the new disciplinary procedure, ideally getting the ECHR to sign it off, and then review Corbyn's position under the new system. Then he either gets fully brought back in, including the whip, or he stays out, rather than this half and half mixture.