I think Erwin would be against the Rumbling. He fought against the titans because they were a threat to Eldia and could not be negotiated/reasoned with. If he would find out that there is a larger world out there I think setting things diplomatically would be an option.
I think in losing Erwin, the corp lost a leader and fell apart. I think Erwin would have been able to do a better job at guiding Erin than Hange did (no offense to her). Thatās why it was a ātacticalā mistake to choose Armin to live. Armin hardly contributed anything as a friend or a guide to Erin afterwards and things fell apart as far as Erin having a person he could look up to or follow.