I have to say I don't remember what he's apologizing here for at all. Is "Red Throne" the episode in question?

I didn't realize it until reading your comment, but I think that theme of Finn's obvious maturity gives me the impression that the author's are preparing to kill the lights.

This is a really interesting question that I haven't seen anyone here ask. Distinguishing between the Betty program and "the real Betty" invites a distinction between Ice King and "the real Simon".

I think you're right, right now it's something we're being told more than something we're being shown. That's not good story-telling. But as these private moments between them accumulate (for example, the dream of Marceline being old and Bubblegum tending her), it becomes less deus ex, ret-con relationship, and more…

I agree it doesn't arise naturally from anything in the story to date, but given their incredible life-spans it's not hard to imagine they've had many experiences, and been very different over the course of millennia.

Adventure Time occasionally explores how we treat non-human animals, but this mini-series was pretty explicit. I'm disappointed it wasn't more so, but I'm glad to see this theme recurring.