1. How do I make the dash unavailable for a determined number of seconds
Use Timer behavior. Before starting the dash, check that timer "cooldown" is NOT running. When starting the dash, start timer "cooldown".
2. Player.Timer.CurrentTime("cooldown") will give you the elapsed timer time.
So, for example, if cooldown time is 3 seconds, you can decrease the dash bar from 200px to zero like this:
Bar Set width to ((3-Player.Timer.CurrentTime("cooldown"))/3)*200
1. I don't know how to implement the items in such a way that it spawns precisely on top of the concrete pool flooring.
The easiest way is to add invisible "Spawner" sprites where you want your items to be created. Then pick a random Spawner and spawn an item.