I do not dispute this at all. As you quoted me saying- the Gigas of Mor Dhona worship the CT. I did not reference the other tribes present ;P
However, as none of those other tribes are in particularly "icy" zones, I would put forward that the likelihood of them being behind Shiva's summoning is slim.
But hey! Yoshi said we will see the beginnings of Shiva's arc in 2.3, with references being made to her in the main story :) So all in good time!
