I'll be glad to answer them Nux
1). Surprisingly there isn't really a worst or useless professional there is probably an even amount of each profession on the server.
2.)
Armor Crafter
-"Skilled Miner" can extract ores much easier and can even find minerals that the untrained eye wouldn't notice! With this mastery, you will also learn the recipe Piece of Hi-Steel.
-"Protector" knows how to make the best use of their armor! With this mastery, while in battle, there is a small chance per attack to recover 15% of your max health instantly (for physical damage dealers only).
-"Juggernaut" is unstoppable! With this mastery, you will learn a new spell which have 50% chance of lifting slows without any cost or penalty! It will cost you 10000 gold pieces to learn this mastery
Weapon Crafter
-Skilled Miner
-Weapon Master = 10% to double your crit chance on your weapon.
-Brute force = 15% more dmg
Potion Brewer
-The herbal master knows best how to utilize the healing properties of herbs! With this mastery, the ammount of health/mana recovered with herbs is increased.
-The Skilled Gatherer can spot even the most hidden herb on the wilds! With this mastery, your rate of gathering herbs and blossoms are increased, and also learns the recipe Superior Elixir.
-A potion brewer's body is highly resistant to potions' side-effects! With this mastery, the duration of potions' penalties are halved, without affecting the recovering.
3.) What do you mean by self cast? There are buffs that you can use on yourself or others.
And not sure about the last question, you'll have to ask Siegh about that.