Hi there,
Thanks for the feedback and kind words.
At the moment, you can only have one upgrade at a time. You are not the first person to point out the limitations around that so expect that to change in the next version :-)
The green "drone" kills enemies when you upgrade it - it's called the Chmmr. You can also upgrade it in the upgrade screen there, try it out!
On the movement, I am considering adding a reverse thruster in the next release as well, will let you know when it's available!
Thanks!