I really like this! It's a good take on oldschool arcade games. I enjoyed the different enemies and how each one has a tell when they're going to come onscreen. It makes it very skill based rather than being about luck.

Needs Improvement:
I think the graphics and animations need work. Destroying enemies doesn't have much weight. I think adding camera shake or maybe some warp effects like geometry wars would add a lot.

It's hard to tell how many lives I have left. I'm focused on the game, so looking away to check lives is a little jarring. It's also hard to tell what level I'm on. I think if the level number comes up overlaying the game during a small break, it would feel like I'm achieving more.

Answering your questions:
1. Determining where enemies are coming from is pretty easy and fair.
2. Yes, I was able to tell which enemies did what immediately.
3. No. The game felt pretty balanced. My only suggestion here is to have a delay between levels to give a small break to the payer.
4. I had no problem clicking the fire buttons, but I can definitely see some users having trouble. I think an option for toggling fire would be a great accessibility feature. Then tapping each arrow button would just change the firing direction.
5. I think a special action could unbalance the game. Adding in a powerup that has to be collected with a time limit would be beneficial.
6. Level 3