Not sure if you already figured out the answer by now. But here it is, I think it’s about him being the one who’ll do the prayer at Yoshito’s funeral (kinda his way to say he’ll be with him untill the end). That time he told Yuuji in the cave that Yoshito probably couldn’t remember it.
i think he said “i love you too” to him probably
What he told Yuuji while they were stuck in the cave. That he was going to read the sutras at Yoshito's funeral. Kenji was declaring that he was going to be with Yoshito for life :)
Yes!
What kenji whispered to yo in there first tym