I think the jet pack could also give you a small boost mid-slide, but if you hit a wall whilst jetpack is on, you get stunned for longer, negating the Get Up upgrade. Controlling the mouse with your sword would seem kind of difficult, considering you're sliding along the ground pretty quickly (as I imagine, you are going pretty fast). Another cool little detail could be sparks bursting from the pixels that touch the ground, for a little more aesthetic. Finally, I think you should be able to turn just a tad bit, because you could probably shift your weight in a manner that would cause you to turn. Possible upgrades would help with steering, and could lower energy use of the thruster while sliding.
1. Yes, the small boost is a good idea.
2. It's meant to be hard, I even mentioned that in the middle.
3. The sparks should be able to be turned off, I imagine that they will cause lag.
4. Yes, but the turning could be with the A & D keys, slowing yourself down with the S key, and boosting with the W key.