Great discussion as always! I love this story. A few thoughts, with the usual caveat that I haven't read any Wolfe scholarship or interpretation beyond the podcast:
The name "Cutthroat": whether or not the protagonist is a human or native of the planet (and I agree with you that he's human), in this ecosystem of northern totems I thought of a cutthroat trout or salmon - a creature whose great purpose in life is an arduous journey to lay the groundwork for the next generation.
Along those lines, I agree with the reading that on the symbolic level, this is (like Pilgrim's Progress) a story of the soul striving to return to its home in God. That moment where the protagonist realizes that the Great Sleigh can hear him rings strongly of a mystical experience of prayer.
And I do see Mantru as a descendant of a much older human presence on the planet, rather than a failed questor, although that's a nice interpretation as well.