It's easily counter-able. The reason they mirror it is because the speed at which to get there, if you run a counter you're more than likely not going to be able to compete with the speed of return. Which is essentially everything for KOTH. Give it a few tournaments, and people will be able to poop on it. I think ESL was the first tournament where they started bringing out that strategy. Reaper will shit on Twinstons, even pump out good damage w/ his blossom against 2x Lucio/Tracer, if he can clear/time against winston shields.
As for the Mercy nerf, she is amongst the three I think we will see being nerfed. On that, Pharah and Genji come to mind too.. I just wouldn't be able to start on how. Though I think if other characters got buffed, Genji/Pharah might not need a nerf. But for Mercy, they either need to do one of two. Either A) Make her ult cancel-able, I can't tell you how many times I've dropped a Mercy at "HER..." and the whole team is revived. Or, if they decide to keep that, B) Remove her ability to cast through walls. Her up time on her Ult I think is fine, because it charges based off poking. If you sit there and poke, you just give her a free ult.
As for the Hidden OP on Zenyatta. Yes and No. Since Double/Triple tank is basically Meta, he essentially does shine to a degree but a single tracer or genji can make it a really bad day for him. Ontop of him not being able to solo heal as well as the others. He requires a lot of communication with teammates, everytime I have someone on my team who plays Zen, I try to ask them to call out their discords for focus. He just needs some tweaks for QoL.