Oof, I've gotten behind.
Let's start with the basics. Huge Outsider is good. I agree it would have the native subtype on Maelost, but don't statblocks usually give the Material Plane subtype?
By that argument wouldn't every conversion of a Maelost creature we do have the Extraplanar subtype? Most of them never leave their home plane anyway,
Also, although the demiplane of Maelost is floating around the Ethereal Plane in its own spatial bubble,
Tales from The Infinite Staircase says Planar Magic treats it as being Prime Material.
Tales from The Infinite Staircase, Page 44
Maelost does not affect spells or spellcasting in any way—even clerics cast spells as though on the Prime Material Plane (that is, at their full levels).
That said, I mainly gave the Dweller the Native subtype because, while it is an unfathomable entity the size of a small moon, the adventure's flavour text calls it "another sort of life" which suggests it has at least some biological processes equivalent to normal Prime Material creatures (in a "It's life, Jim, but not as we know it") like needing to eat or otherwise metabolize matter to sustain its flesh.
It didn't feel like the embodiment of a pure (or impure) spirit like a regular Outsider.
HD and physical abilities seem reasonable for a tendril/portion. But it's really part of an epic kind of beastie.
Upon reflection, I was being extremely conservative with at least one of the physical ability scores. The original adventure says the Dark Dweller's tendrils pull and hold characters with 25 Strength.
That's Strength 25 in AD&D terms, which is Titan strength or the highest the standard Str stat goes, exceeding even the strength of giants, the standard AD&D giants going from Str 19 Hill Giants to Str 24 Storm Giants.
In third edition, a Titan has Strength 43.
That seems a bit extreme though.
What would you say to boosting the mental stats?
Sure, why not.