Not sure what’s gone wrong here, although there have been some issues recently more generally with pip (Python’s package index manager), perhaps this is from another case of that.
It seems like for some reason the future module isn’t installed. You can install it by typing
sudo python -m pip install --user future
into the terminal, which might fix your update issue, but if this is due to some underlying problem that’s occurred at some point in your various update/downgrade attempts then it’s possible there’ll be several more issues after it, in which case your best bet is likely to flash an image directly (which unfortunately requires opening the enclosure). If that does end up being the case, you may as well update the Pixhawk while you’re at it.
If the future install doesn’t help and you’re very against opening your enclosure then the software team may have some further insights/things to try, but generally at a point like this the most efficient use of time is to flash on a working image rather than trying to hunt down and fix some arbitrary number of things that may have gone wrong across multiple failed updates.
If it’s helpful, I’ve covered the easiest way of accessing the enclosure electronics here.