You make a lot of good points, I will admit. However I have a thought regarding the ending of Halo 5, and what you call John and Locke's "nonversation".
Their emotional resolution already happened, before Blue Team was abducted into the Domain. Osiris runs up to Blue Team, who react defensively, until Locke puts his gun away and his hand up: They're there to help, not to fight, and they know that Cortana is the threat. Blue Team knows this too, and John hopes that he can talk her down. Locke and his team want to help, but before they can Cortana steals them away.
John and Blue Team are then betrayed; there's no talking Cortana down.
Osiris stops Cortana from imprisoning Blue Team, but the Spartan II's aren't aware of that fight and it's outcome. So when John asks Locke "Where's Cortana?" what he's really asking is "Is she dead?" But she's not dead, she's gone, and the threat remains. And we're even still unclear if John will acknowledge the threat that remains, or if he'll try to get her to "come back home" again. Does he have the strength, as the Warden Eternal mocked, to crush her in his armored fist?
What's more, there's even a secondary meaning to what Locke said: "She's gone" could also be taken to mean "There's no saving her." She's no longer present, and she's no longer the Cortana that we knew - if she ever was.
In the beginning of Halo 5, we see that Chief is not over Cortana's apparent death. He ponders on his helmet, stroking the AI Chip slot. He leaps at the opportunity to find her, going AWOL to do so. He strikes a fellow Spartan to keep them from getting to her first. By the ending, he's realized her threat. He's held her accountable for massive losses of life, and quite likely has emotionally let her go to address the threat that she now poses to the galaxy. She's gone, and it took John long enough to realize this. That's the journey (for John) in Halo 5, and it's not cliffhung at all.
That's my take on it.
