Personally, I like what they did with Abeloth.
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I didn't even like the Mortis Trilogy episodes in TCW, but I liked how Denning incorporated them into Apocalypse. I thought it worked well. But I guess it's not everyone's cup of tea.
To me, tying the Mortis characters to the Celestials and then back to Abeloth made sense, and Denning left a lot of wiggle room for the details to be left to interpretation. Remember, the Killiks memories are shared things that assimilate myths as well as facts. And just because Luke thinks something doesn't mean he's right. It certainly wouldn't be the first time he's been wrong.