How does one effectively understand & navigate through the suicide cases found in the canon?
First of all you are equating the breakup of an Arahant's body to the breaking up of the body of a non-Arahant, the two are not the same.
Death is spoken of a as a requisite for birth. There is no birth for an Arahant, this is a difference. An Arahant doesn't die, with the breakup of the body they become completely released from all modes of being.
You are also slipping by the fact that Arahants are more or less the same in mind, different only in body.
It's not like one Arahant is depressed and kills himself whereas another is more psychologically resilient.
As i see it, Ven. Channa used the the knife because he considered that his being there wasn't needed as he was gravely ill. He had done his work here and had every right to end it as he sees fit.
In some circumstances Arahants do choose to go early but that is as far as it goes.
As for Ven. Nanavira and Ven. Samahita, i have little to no interest in reading works of disciples and will probably never know what Nanavira wrote in his notes. I just think reading the words of the teacher is superior that's all.
I was sympathetic to Ven. Samahita due to him always taking the time to pay homeage but he also said some strange things like 'consciousness being like radiation', went on that anti-muslim rant earlier due to a medicine and later killed himself.
Whether he was this or that doesn't matter to me as even if he was an Arahant it is of no help to me now.
As to controversies, imo the Sutta are explicit enough for anyone to make up their own mind and even writing commentary.