Don’t worry about staying in the upper half. That’s completely natural for a beginning player because it’s much easier to feel “in control” of where the bow is going.
All of your concerns are addressed with the same type of work – wrist control. You are right on track with your idea of flexing the wrist! As the bow moves up toward the frog, the wrist should arch upward. This allows for more control, as well as the lifting of pressure as you move toward the heavier end of the bow. That is not only the heaviest wood because of the frog, but your entire hand/arm weight live there too. Conversely, as you are bowing down the wrist should straighten back out, then turn upward as you approach the tip.
An unflexing (tense) wrist will cause your bow direction to follow your arm direction and create what I like to call the “banana bow” effect that you’re seeing in the mirror. When you’re moving up and down, think of your arm and wrist and flexing then straightening instead of moving up and down. Give that a shot when you’re looking in the mirror.
Also, go to YouTube and find your way to the Violin Tutor Pro channel there. Michael has some good quick tutorials there so you can visually see what I’m talking about.Good luck!