Nice find! Will try to fix this goofy AI :D
1. Car whistle/horn - I agree it should be in the game! Lots of potential uses for it.
2. We have plans for special abilities/items that can be mounted (like parts) and used. One of them will be some kind of nitro boost. These will come a bit later in the development though.
3. Yeah, current free roam is like first, bare-bone implementation. We added it primarily to slow down pacing a bit between fights. Will be adding more meaning to it in the future updates.
4. Some AOE weapons are planned, we just need to work a bit more on what we currently have in the game first :D