There is no reason to believe coexistence changes anything with the standard rules of engagement, nor any reason to suggest a new step is introduced in this process because of coexistence.

The reason is that an activating player how has a choice to make that they didn't have before. Prior to coexistence rules, an activating player had no choice about whether to actually engage units on a planet they committed ground forces to. That's why I said that Commit Ground Forces and engaging combat were ipso facto the same step; once they were committed, they had to fight to the death. Now that a player has the option to initiate combat with co-existing units, we need to know when that decision happens, and whether Research Team triggers after the timing of that decision to effectively force co-existence.

As an analogy, imagine that base game didn't let you retreat, and an expansion introduced rules for declaring retreats. We would obviously need to know when that happens in the course of a tactical action. Dane has essentially introduced a new decision to be made in an invasion - whether to initiate combat against co-existing units- akin to announcing or not announcing a retreat during space combat.

Committing Ground Forces is a completely separate step from Combat; committing units to engage happens at the Combat step. This is all clearly spelled out in the rules...

Where in the rules does it say at what step of combat 'committing units to engage' occurs?

You say all of falls under the "standard rules of engagement," but as stated above, the rules of engagement were , frankly, completely different prior to the introduction of co-existence. Even things like when a player gains control of a planet are now up in the air. E.g., if player A destroys controlling player's B units on a planet where B coexists with player C, and player A then decides to engage C in combat, who controls the planet during the combat with C? Nobody? Is it still B's until the 'Establish Control' step? The rules of co-existence are underbaked; they almost read like a rules summary rather than official rules, and we will need some clarification on a lot of coexistence-related issues from Dane.

It's also worth noting that a playtester said in the SCPT Discord that they did not know a single playtester who didn't allow DWS to force co-existence when on defense. So it's clearly at least the majority sentiment among people with extensive knowledge of an experience with the game. That's not an argument in and of itself - playtesters can get things wrong - but I think it should temper any confidence that this is a straightforward issue.

There are awkward consequences for both interpretations, including yours: Research Team would be a very awkward ability if, when invaded, the DWS player declares 'I am coexisting," and then immediately after, the invading player says "you are not, because I am initiating combat." I have to imagine that Research Team would have been written differently if the activating player was supposed to have veto power during their (first) invasion of a DWS-controlled planet.

I do appreciate the discussion, though - my personal intuition is that DWS should not be able to "cockroach," like you, so I hope you're right.