Technically they are very similar. Hence why I say they are similar.
Pop isn't a legitimate genre. It's a referencing word, just like "grunge", or "indie". Most "pop" uses it because it is in fact rock. Musicologists have known it for years. Hence why people like Michael Jackson and country stars like Hank Williams Sr. are in the rock and roll hall of fame. Because, in reality, they play rock music.
You are strawmanning again. Rock isn't defined by tempo, or time signatures, or solos (namely because through variations in notation any song can be rewritten at any tempo and come out the same). Thousands of bands play wide variations of those. Instrumentation simply refers to the instruments common to the genre. Sigur Ros uses guitar, bass, keyboards and drums predominately. It doesn't mean what they are playing is rock. It isn't. They use non-rock chord progressions, rhythms, and harmonies. I've heard people argue that they also use non-rock melodies but that is a crap argument because rock isn't defined by it. So technically it's not rock. Mogwai on the other hand was rock at times, but not all the time. It depends on the song.
No. What influences can be incorporated into rock's objective definition is objective. Once those boundaries are broken that isn't rock anymore.
What do you think is actually original about that album? Sure it's cool, sure it might bring some of elements of Native American music to a broader audience, but it doesn't break any new ground. Native American music has been borrowed from for many years now, just as Egyptian and Oriental music has been done before.