Why is the emotional spectrum limited, if Volthoom can recharge by being an Emotion Eater? Because for the spectrum, recharging means taking emotions back. Or, in the Entity's case, taking life back. It's a catch-22: Letting it run out leaves everyone physically dead, but recharging it means leaving everyone emotionally dead (and possibly physically dead anyways).
Yes, killing the Entity was Nekron's plan. Here, it dispersed its energies through-out the universe, re-filling the spectrum. The main difference is that the Entities technically still exist, but are scattered far too thin to reform.