My own rewrite for Michael being "stuck" on the island. Didn't like that he was stuck for killing Ana Lucia/Libby when Ben killed so many people, including his own father, Dharma Initiative, Locke and he gets offered a chance to lead the island as #2, and maybe move on as seen in flash sideways?! Doesn't make sense. If anyone is redeemable, despite their past, Michael should not have been stuck on the island.
In theme with Jacob's perspective that one's past does not matter and that you always have a choice, Michael is on the island by choice. He choose to go on the freighter to redeem himself. When he died on the freighter, he remained on the island to be able to advise the candidates and future island protectors based on what he knows & sees. That is what the whispers are, not tortured souls but those who remain to advise. Those with a gift, like Hurley & Miles or other candidates & their allies, can hear them or sometimes even see them, like Sawyer was able to see a dead young Jacob. These spirits of those who died on the island or in its sphere, like the freighter was off the coast & was impacted by the islands' power, remain as part of the way the island protects itself. While there is one official protector, like Jacob, Jack, Hurley, it is a team & generational effort in this way.