1. That is just there to identify enemies of the same type among one another. The letters are the same for everyone in your party so you can just tell people to attack Enemy Z and Z will be the same enemy for them too.

2. The symbols are for agressive, non-aggressive, and I believe there are two more for quest-only enemies that you typically have to weaken with an item. That one has an aggressive and non-aggressive variant.

3. Aggresive enemies have a red icon to the left of the nameplate.

4. "Others" and "All" refers to your targeting filters when using a gamepad. By default, the game will switch to the "Others" filter when you take out your weapon (so you can only cycle through NPCs and Enemies), and change back to "All" when you put your weapon away. The HP/MP/TP at the bottom of your screen is always your own.