[Coldsaturn]

Isiah defecting was a spoiler for all of us, it's literally the new lore drop of this card! Can't remember now if you knew that Isiah was working with them to begin with though. But congrats, I think this is the first time you learn something together with the fandom! ;_;
Since it was a LI-specific card, you get the fully unadulterated version of their dynamic compared to main story Xav which is slightly more neutral. So in this card they're already early romantic with each other. And yes to MC throwing in his face that he just won't tell things to her. Good for her.
Can't help but agree on the angst, too. "what if I die? Will anyone notice?" is TRAGIC for Xav considering his time-travel status where people are literally not supposed to know he exists for long (or they're going to figure out he's near immortal). But all Xav's cards tend to be more subtly sad than outright angsty.
About how much MC laughs it off rather than understand the full implications of what is being said, as of now that's part of the angst for all the LIs with more memories than she has. I think Zayne is the only one without that baggage.

Your interpretation of the collar is on the positive side, like something meant to protect him, but let's also remember when it appears. It's when the first MC he meets (school classmate) is dying and he disappears for a long while looking for a specific protocore that could save her. When he comes back with that protocore it's already too late, but he has the collar on.
And that collar has ever's logo on it and is a suppressor. Knowing ever, should we think it's a collar meant to protect him from using too much of his evol? Why should it be a problem suddenly when he was using it apparently to its full extent before that mission for the protocore? I don't have a definite answer here, I'm throwing info as I remember it.

Yes to side effects to being away from Philos.