At some point a lot became fixed, but some in the wild may still have the loose screw problem. I don't know the fix, but I'm guessing someone here will have info on that.
The second included screw is different, and was an add-on to give the two output options. Mine only came with one, installed, I don't even know which one, I just use the thing. That's the short story I can offer there.
When inserting a battery, I think most of us feel more comfortable with positive end down, it is better protected and if there is a venting situation, it is pointing away from you!
Modulation of the button is key to happy vaping with Mi3, but other factors, especially your draw speed need some attention as well. Loose track and you will char material...
Get your technique down and use as you like. I enjoy spending a long time with a chamber, many clouds without super high temp. Others rip it in a few big pulls; it's up to you.