you may squeeze a shaft collar in there if you're lucky. Though you're right, there doesn't look like there is much room. I use a collar that's about 25mm wide. They may come in other widths.

I don't think it will matter if it touches the prop. There needs to be an electrical connection between the prop and anode anyway, which is made by the prop shaft they are both bolted to. If you have a multimeter, then once you've fitted the anode, use it to check for continuity between the prop and anode. That's what I do, just to make sure everything is ok.

The collar anodes aren't as big as other types, such as the ones that bolt on the boat and mine dissolves very quickly. if you get the opportunity to dry out, then check it in say August and see if this is an issue. You may need to supplement it with a type that hangs over the side of the boat, which you can lower in the water when you leave the boat on its mooring. That's what I plan to do anyway. Of course if your boats a bilge keeler you can easily keep an eye on it and replace when necessary.

As for the material, I thought they were all Zinc. Different types, like magnesium, are available, but I thought they were for use with different boat materials, not sailing areas. Though I may be wrong.

Oh and, you may be well aware of this, so appologies in advance, but don't antifoul the anode or it won't work properly.