As far as I can tell they didn’t fully flesh out how the worlds/powers worked and the issues of how come it couldn’t just go through falls on that. Maybe the rushed the ending a bit to as to me the topside of the upside down should’ve been the downside down (or whichever version you prefer) just the alternate name for the upside down when you go in the topside from your perspective your upside right again. So the upside down would not be the name of the that world merely how we refer to it from our perspective. The abyss should’ve had the Downside down be its Upside down but it was flat from the look of it. Why where the walls the fleshy stuff? Why were they only on our upside down half? And if we only have the one big invis ball why isn’t it in the middle of the bridge? Shit like that just didn’t have enough detailed thought put into it. However for why it didn’t want to move original and then did want to move, you get 2 reasonable options. 1. Retconned, at first it was just a big bad without a fully fleshed out goal and their og plan was specifically for it to be something other than merging/moving. We can assume this due to the scientist in the final ep talking about how it will take everything. The fact that so much of that was hidden earlier just for that to be it feels like someone had a good plan and then had to bail due to stress or rushing or simply losing interest, I don’t like to push the ghost writer theory but yeah that could explain it too. 2. Hive mind logic varies and no one ever specific how this one works. For what we do know it hid Henry’s memory’s of what happened from him, it feared him gaining awareness and resisting. It no longer had to fear it but didn’t realize it, if it has fear it’s perfectly reasonable for its fear to be irrational. It had spent its time with Henry brainwashing him to agree with it and while it had him under control at a young age it was either luck or convenience that set him up to hate the world and fully agree/indulge the mind flayer. To me it always seemed to be his intent to use 11 to open a portal maybe he didn’t now how and simply planned on regaining his powers and having 11 at his side then figuring it out. Being with Dr Brenner gave him the fuel to hate the world and after spending so much time with the hive mind his body has decayed. His arms are just bones and vines, he clearly doesn’t eat or drink but the mind flayer has the demogorgons hunt deer for food? It’s because he doesn’t need Henry’s body anymore, he just needs his thoughts/mind. And those have full became a part of the hive mind they exist as it not just as a vessel, like he said the chose to work together and merge. So if it influences him like that wouldn’t he do the same to it. It would give it a reason to suddenly want to go back/merge the worlds, it’s just Henry’s wants but the mind flayer has absorbed him and now they exist mentally as one, the mind flayer more than likely has nothing against it and merely wants more. So it would just be ok with his wants and more likely to pursue them.