Hi, for the water in the footwells. Have you looked at the front firewall. usually this is the area that you will find items attached to and the seals may get perished or corroded such as rubber seals.
Not sure on the micra exactly what is in this area but i do know that the brake res is situated on the drive side at least in my case.
if you were to lift up the carpet or fold it back at the pedals and get a torch in you might be able to see the area where the water is dripping from other than this it could be entering in another location
Another thing to consider is to check under the wipers. Remove these and the faceplate. Then check that this area is clear and that the water runs off. so you know it does not stand in that area if its full of leaves and debris this can also cause damp inside the cabin.
I've not had this problem on mine but i did notice the other has had the top area blocked in summer.