How to wire a smart switch - 2 gang switch box?
I'm going to disregard some of the questions about the existing switch because they aren't relevant except for purposes of learning, and I'm not sure I understand them clearly. You can simply remove that switch because you know the hot comes from the other switch, and the load is the remaining black which presumably leads to the fixture.

You're on the right track except that your assumptions in number 2 are incorrect. You would not remove any of those whites from the bundle. Doing so would probably leave you with an open circuit. You'll need to add a pigtail from that bundle to your switch, which requires a neutral, whereas the original switch did not.