There was a Mega Raid about to happen near me and as I needed to battle in a raid for one special research task and battle in a Mega Raid for another special research task I decided to kill two birds with one stone by going to it. Unfortunately while the battle in a raid task was completed, the Mega Raid one wasn't. I was the only person at the Mega Raid and so I guess for the task to activate, either there has to be more people or I have to actually defeat the Mega Pokemon. Even if I could, it'll require using up a bunch of revives and healing items and thus use up a lot of time. That's a bad thing as when you're looking for a Rocket Balloon, whenever they do appear you have about 20 minutes to engage them in battle and beat them before they fly away.
So it turns out I made the right call in backing out of the Mega Raid once it defeated the first team of Pokemon I sent after it as to my luck the Meowth Balloon finally appeared again. Not only did I beat Jessie but got my sweet revenge by beating James too. What's funny is the first two times I encountered that balloon, it was at home, but for this final time it was outside in the cold and snow. The thing is that the final part of Grovyle and Celebi's journey took place on a snow mountain, so that was an oddly appropriate battle with Jessie and James. So yeah, I've now fulfilled all the requirements to unlock the Shiny Celebi encounter, at the moment I'm currently preparing myself for it.
I also got another 7 km egg, this one from Dallas, Texas.
EDIT: Story of my life, the Celebi encounter requires AR. The problem is that I've been having AR issues, so that's something I have to fix ASAP.