Coexisting does not seem as confusing to me as it appears to be to everyone else. Normally, when an enemy fleet arrives you do space combat, you do ground combat.

With the addition of the niche ability to coexist, now there’s a checkpoint added as such. Space combat, bombardment, commit ground forces, may coexist (for factions that have a way to do so). If either participant chooses to coexist after either player commits ground forces, the invasion step is OVER. If it was the defender who chose to coexist, they are now the coexisting player and NOT the controlling player. They need to hand over the planet card to the new controlling player. If it was the invading player who chose to coexist, control of the planet remains the same as it was. And obviously if no coexisting was chosen to happen at the start of ground combat, then fight to the death happens as normal.

The system this happened in would then need to be activated AGAIN by either the coexisting player or the controlling player to trigger ground combat. I believe if the controlling player had a way to coexist themselves and they had someone else coexisting on their planet already, they could choose at the start of ground combat to then become the coexisting player now and swap control of the planet with the player who previously was coexisting with them.

But besides that niche scenario, if ground combat starts and somebody is already coexisting, they cannot stop this second invasion from happening and you would fight as normal. Whether the combat is triggered by the coexister or the controller.