1> You should have a fork on the top of the mast where a lump attached to the halyard slip into to lock the main in place. Once you pull on the downhaul the halyard is not going to move out of the fork. I have a small cleat on the starboard of the mast for it, but I usually do not keep any tension on it.

2> DO NOT KEEP IT MOORED. With the mast up it will be quite instable in the wind, and no matter how you attach it it may capsize by itself. In addition keeping it in the water all the time will induce osmosis really fast. Bad idea. Get it out of the water.

3> The mast base should be held in place by rivets, not screws. Aluminium rivets are probably fine there, but I'd go with Monel ones, they are more solid. Don't put anything like stainless, it will corrode the mast and/or the mastbase very quickly.