For making sauces and dressings XG, GG and LBG have different qualities. Especially in lower concencrations my go-to thickener would be XG however as you start using more of it the texture becomes “snotty”.
GG on the other hand, gets more sticky as it gets more concencrated. To balance each other out you can mix it to get better thickening results in lower concencrations something like (0.5% GG + 0.35% XG). I think this answers your 1st application and 3rd question.
LBG and XG works much not only as a thickener (lower concentrations), but also as a gelling agent when used in slightly higher concentrations (0.8% LBG + 0.2% XG). You can try using this syneresis to achieve a texture for your second application.
I don't have any knowledge about using Xanthan, but I have used guar gum quite a lot in the past. So, I will stick (!) to guar in my answer to you. First things first, you don't need a lot. Try sprinkling a small amount (I'm talking no more than a coffee spoon to start with) and quickly mixing in, this should then show you if you need to use any more. It is a great thickener, and suitable for celiacs.