To stop a character when you shoot, I believe disabling the platform behavior is the easiest way. I would set a variable called 'Fire' as true/false. When the player presses the shoot button, set it to true. When this variable is set to true, create an event that disables the platform behavior. Then add in a wait() command through System for a second before setting the 'Fire' variable back to false. This will re-enable the platform behavior.
On your second point: This is covered the beginners tutorial (specifically on page 5, under 'Get the Player to Shoot'). You should adjust an image point so the shots originate from it. I was going to include a link but I don't yet have enough rep to do so. You should be able to find it via Googling fairly easily. Or if someone else could link it, that would be great too.
If you would like clarification on anything I just mentioned, feel free to ask. You might also want to consider using DropBox in the future for sharing your capx files. It is more convenient and trustworthy, and from what I've seen it is the standard around here.