Do we know, for sure, that Av-Matoran do not have a shred of heat resistance? And that Takua did not suffer whatsoever from discomfort?

Maybe 1,000 years of life in Ta-Koro allowed Takua to adapt.

Do we know, for sure, that Av-Matoran do not have a shred of heat resistance? And that Takua did not suffer whatsoever from discomfort?

Heat resistance is a Ta-Matoran thing. Av-Matoran have the ability to change their armor color and launch bolts of light energy. And he said himself that the only place he felt comfortable in all of Mata Nui was Ta-Koro.

Elmara

Note that Takua only lavasurfs twice, whereas everyone else seems to be doing it all the time. This would probably be why.

An interesting point, but also remember that we've only seen Takua lava surf, but we know that others do too. The two times we've seen Takua surf likely aren't the only times he's surfed.

Elmara