1. Warm Tone papers have an emulsion that develops "a little warmer". This difference is usually light. You can tell if you have two prints side-by-side. To me, the most important characteristic of WT paper is its ability to react to toners. Where as most cold and neutral tone paper react very little to toners, warm tone papers react dramatically. Ilford FB WT does have much creamier base paper. Some WT paper do not.
2. Yup.... each paper is different. It's what you like, most of the time.
3. I don't think FB singly ply exists.... but it's hard enough to handle wet FB double weight paper, I don't think I'd want to try thinner paper....
I use Ilford MGIV FB, MGIV FB WT, and Adorama's house brand paper. It's glossy fiber version is a neutral paper. It's matte version is warm tone. The paper base is white but the emulsion is quite "warm" in that it reacts to toners very well.