2. My favorite mulching materials are shredded leaves, especially oak, followed by compost and ground fir bark. We have tons of pine needles available here (Ponderosa pine) but few use them except for winter protection. Black mulch can be made of any number of things, like wood chips or bark, and is dyed black. It can sometimes steal nitrogen from the soil as it decomposes.
3. I don't know if the growth of the palms would be retarded directly by the fabric; the fabric inhibits interaction between the soil (and all that lives there) and the world above. That deprives the soil of organic replenishments as the roots and critters consume them, and eventually the soil begins to die. THAT would retard the growth of your palms.
4. The fabric alone (uncovered) would slow the cooling of the soil somewhat, and would hasten the warming of the soil during the day. The trade off isn't worth it.
5. One of the selling points for landscape fabric is water conservation. Preventing drying winds from sucking moisture from the soil leaves more available for other things. A mulch does the same thing. On a heavy soil, overwatering would cause root problems.