There are at least two thicknesses of water-soluble stabilizers known as Vilene. One is soft and one is stronger and also wider (500mm wide). I use two layers of the strong Vilene, as I have tried the three layers and it was way too much. So it depends on which type you use. There is no need to set the two layers at right angles if you use the stronger Vilene.
Yes, and here's how I do it.....
I use three layers of WSS. Top and bottom layers are set at right angles to each other and I fill up the centre layer with scraps of WSS cut from previous projects. This gives a good base for the FSL stitching. I have never had a failure using this method and I learnt it from Phil of Bargain Embroidery in a Work Along a couple of years ago - her embroidery is just wonderful.
Some people use two layers and some people use one. I go for the three layers every time. That way I can choose to have firmer bookmarks or coasters, or take more time and soak the FSL until it is soft.