If God says, "What more could I have done," then who are we to say HE must do more, until every last created sentient being is brought to love and salvation???
I believe the all-powerful Creator has limited Himself. He WILL NOT violate the human mind/will. He will not use mind manipulation. THAT is where He has drawn a line - for Himself.
Everything else - short of that - He HAS DONE. He has used "tough love" allowing human beings to experience the consequences of their choices. He has rescued them. He has taught them. He has revealed Himself to them. He sent His Son to become a human and then die by torture (a sacrifice beyond our full comprehension).
Israel and what happened to Israel, is a MODEL - a TYPE, for what God has done for the whole world, and how God will deal with the whole world.
Just looking at that MODEL, almost all of those who left Egypt (who had been slaves) DIED IN THE WILDERNESS. They resisted the Holy Spirit, forgot all the miracles, and were "broken off" because of unbelief. They could not deny that God existed. They just doubted that He would defeat their enemies and give them the land HE had promised.
I believe this is the BOTTOM LINE argument. Universalists keep saying "He is responsible" therefore "He must do more" until HE has saved everyone.
I say there IS NO MORE that He can do, without violating man's free will.
In the MODEL: When divine "fire" consumed Nadab and Abihu - it killed them. It didn't "cleanse them" so they now behaved righteously.
When divine "fire" consumed the 250 princes - same thing.
I don't buy the argument that the fire is just "cleansing". It cleanses "the camp" - Yes. But it cleanses the camp by consuming "the chaff", AFTER the wheat is harvested.