Is it possible for toa to remain in a kaita state permanently if they choose to? Or are the forced out of the form passed a certain point? For example could the nuva had stayed in the kaita state after battling the Manas and Bohok Kal?
I am no Greg, but I can tell you what I and my spouse headcanon.
Each Toa Kaita is formed to perform a certain Duty. If the Duty is somehow remains unsolved over extended periods of time, it’s quite possible a Kaita would stay fused.
For example, In an AU we’ve been developing together, there is Kaita of Vengeance, a naga-like Fusion Spirit that protects a sunken temple from desecrators.
I don’t really think headcanon will make up for an opinion of Greg’s, but possibly make it harder for him to answer the question asked.
But I digress.
He will have his own opinion. =) Besides, given that Kaita is such a heavily underused and underdeveloped subject, there’s not really anything solid about it, unless Greg says otherwise. Maybe he’ll think my headcanon will make sense? Anyway, I do not even dare to think that my opinion overrules Greg’s, and I hope it won’t hinder his ability to respond to your question. =)
I think in different forms of the Manas battle, after they defeated the Manas they accidently separated because Makuta ‘scared’ them or something of the sorts. There was also the time the Bohrok Kal Kaita defeated Wairuha and forced them out of Kaita. But I may be wrong, my memory is fuzzy on that battle. But it mainly seems that they just have to retain focus and unity.
It is stated that the Makuta possess an ability to break the fusions at will. So in Mangaia, Teridax did just that.