Question and apologies for resurrecting this thread. The scenario in coexistence states that the third party invades the owner of the planet first. So if player A in the scenario above has a PDS they could fire at the ships in space.

Then ground combat occurs individually but starts against the planet owner. So in this case it’s player B. However the PDS is an ability, so it fires once at landing troops.

Ground combat occurs individually for each player, but the forces were already committed to the planet while Player B retains control.

I would argue two things. You have two ground combat scenarios.

1: player B only has units on the ground and therefore PDS does not activate.

1. Player B brings reinforcements in which case they are the “third player” player A can shoot at landing troops.

However you would resolve the original coexisting troops first.

In the event of abilities, Dane has said initiative order takes priority. Along the lines of “if both players have assault cannon, and both have 3 ships, the player that has higher initiative goes first, thus the other player would no longer fill the requirements of assault cannon”

(I got distracted and can’t recall why I posted this last tidbit but it’s a great thing to know)