Don't have spinach? Leave it out, add a drop of green food coloring (or omit entirely and have a white mint chocolate chip ice cream), and miss out on the green benefits.
No, I don't recommend substituting Greek yogurt for the cottage cheese; it's too tangy. You don't taste the cottage cheese but it adds a nice creaminess as well as protein.
If you don't have half and half you could try substituting half unsweetened almond milk, half heavy whipping cream. I prefer the texture of using all half and half, though. For a richer result, use part half and half and part cream.
You could substitute whey protein powder for the collagen, but I prefer the taste (or lack thereof) of collagen.
Feel free to use your favorite sweetener to taste in place of the sweeteners I have called for in the recipe.