What percent fat should the labneh be? What's the least % fat you can get away with, without harming teh final product

Personally, if I'm cooking for myself, I use the full fat version, and just make small portion sizes. But this takes mega-self control and not everyone can manage it when the recipe is delicious.

So, cooking for friends and family, I would probably go for no less than 2% (less than that, weird things happen in the manufacturing process, that I personally feel are not acceptable to feed my family). For those concerned with too much animal fat, I might substitute some coconut butter for some of the real butter. I have a friend that will often replace tofu for half the cheese in any recipe.

But please keep in mind, we need fat to live, especially our brains. So why not leave the fat in the delicious treats, and eat a vegi rich diet the rest of the day?

Taste is 100% subjective. As someone who totally lacks a "sweet" tooth, but has always had a "fat" tooth, what is acceptable to you might not taste quite so good to me. My suggestion is you start experimenting so as to determine what works for you.

I wasn't planning to buy the labneh but rather make it using rivka's recipe, which is linked to on the tart page

Regular good quality yogurt made with whole milk is about 3%. A labne labeled 2% most certainly has thickening agents added. Depending on what they are they can either loose their binding power upon heating or go lumpy.

I was hoping I could get away with 2%

My best guess would be about 9%. Though I don't think the final result would suffer with a lower percentage, as long as the water content is properly reduced.

It mostly depends on how creamy you want it. The higher the fat content, the creamier and smoother the mouthfeel. A low-fat (1-2%) version would still have good creaminess but be lower in fat than whole milk labneh. I personally wouldn't use non-fat as they usually add artificial thickeners to firm up the extra whey and are more acidic than low-fat or regular yoghurt, but you could use non-fat if you wished and still get a good-tasting product. I'd stick with whole milk myself, especialy since by using labneh (instead of cream cheese for example) you're already drastically reducing the fat content of this kind of dessert.