The six original Toa as in the ones that were sold beck in 2001? Besides 2002 (or was it 2003?), there has always been six new Toa sets every year, and I've collected all of them.
How well do you follow the story? I've also collected all the books in the Chronicles(2001-2003), Adventures(2004-2005), and Legends(2006-2008) series, which basically tell the main parts of the story.
For your question about Tahu's Hau mask, I think explanations 1 and 2 work, but for where you ask why turn off the mask power, I think I can explain why. Activating a mask power requires some mental concentration. Therefore, it would make sense why it would be difficult to keep it on all the time. It would also explain why it took the Toa Metru a long time to figure out how to activate their mask powers, as they had never used them before, at the time.
I am not sure about the name but I think is Rahkshi Kurahk... I have the white coloured one.
Yes, you have the correct name. The white Rahkshi is Kurahk.