there are two possible reasons for taking out such a big chuck of plot/dialogue (or well three technically i guess) that i can see;...
Yeah - all three of those seem like totally likely options at this point. Though another reason I think they removed it (and this is in line with #3, actually) is that it made the episode too unfocused, and possibly made the revelations present in the final version of the episode and those about Finn’s Father simultaneously undervalued by the other. Like, at that point, too much was happening, and when you only have 11 minutes that’s really easy to do and you have to cut a lot out.
Actually, thinking about it, there were a handful of continuity errors in the storyboard: not just the adopted thing, but also calling The Lich “The Lich King” and stating that Grod (I think it was Grod?) lived in the Time Cube. And, then again, the storyboard that we all saw was an extremely early draft, and they do multiple drafts exactly for ironing out errors like that.
As far as the other stuff, The Lich was very specifically undefeated (in the traditional sense) at the end of Jake the Dog - and, knowing what we do about it, defeating The Lich outright and permanently (and learning such things like what Finn Prime’s lineage is, etc.) is series finale business, or at least near-finale, if it can even be done.