1. Blue Mountain Dwarves can only be found in the Blue Mountains, and Durin's Folk can only be found in the Iron Hills and Erebor. However, the Blue Mountains are MUCH larger. All of these biomes are quite out-of-the-way, the Iron Hills are definitly the most remote, and Erebor is closest to civilization. Both factions are basically identicle, but the one advantage Durin's Folk has is conquest. Durin's Folk can conquer more areas than the Blue Mountains since they are allied with Dale.
2. Both sub-factions are essentially identicle, they have similar content and similar strength. However, Lidon controlls a much larger swath of territory (albeit more remote).
3. Either is fine, Wood Elf scouts are fast, but Galadhrim Wardens are harder to see. Both factions have comparable units and a similar amount of allies. However, Lothlorien might have more conquerable areas since they're allied with the Dunedain (I'm not sure though).
4. Dol Amroth, if you have deep enough pockets.
5. Umbar, in every single area. Although they have a small territory, and the Corsair holdings are out-of-the-way.
6. I don't see why Rohan wouldn't be worth playing as. They have functional villages (not like those abominations in Dale or Angmar), very strong mounted units, and a large territory. Their only weaknesses are their armour and slim list of allies (only Gondor).
7. I've never used mounted units, but they're difficult to fight against, so perhaps.
8. Hitting things from a distance. Archers are better though (and axe-throwers).
9. Athelas is very rare, but it's best found in Gondor. I got about a stack from walking along the roads and exploring around villages when I raid them (I'm an evil player btw). You need six plants to brew the drink (you'll get 16 cups of drink). You can also try useing bonemeal to spawn the plant in Gondor or prefferably Meneltarma (if you can find it), but your chances are slim.
10. I'm not sure, the Black Uruk Warhammer is the best one I'm aware of, you should probably make modifications to it though. It's slower than an old sloth without some improvements.
11. Black Uruks are stronger than Gundabad Uruks and standard Uruk-hai. However, Uruk Berserkers are taller, faster, and stronger than Black Uruks, making them the strongest uruks.
12. If there is any difference it's negligable. The Northern Rangers are more widespread and easier to find. So that makes them better imo.
13. Gondorian Tower Guards are faster and stronger than normal Gondor Soldiers.