General advice is play what you enjoy. As of now this game is balanced fairly well and everyone can be a viable contributor in any content. Some classes have to work harder then others, but everyone can fit in if they work at it. Why does any of this matter? It doesnt, its just a reassurance that once you find something you enjoy you can make it work.

1. So when you say "teaching me mechanics", do you mean the mechanics of the class in question or game-wide mechanics? Im going to answer as though you mean non-class specific mechanics. Id give the edge to the casters/bard. Its easier to see the thing you need to dodge if you dont have an enemies behind right in your face. You can get a leg up on this sort of learning by completing the Hall of the Novice training at level 15(accessible from any "The Smith" npc, or from its physical location in outside Aleport). In general, the game is decent about not throwing everything at you all at once. Early dungeons and trials have simpler things to do/avoid then later ones.

2. Sure. Lots of people pick a tank first. "Leading" in a dungeon you cant get lost in isnt really that hard. Tank advice is something like "point out that youre new and take constructive advice, take it slow until you learn to go fast, learn to control hate and position enemies, learn when/why to use your defensive abilities(also, USE THEM - a tank afraid to use a defensive skill is a sad sad thing), and finally learn when you can switch from hate generation to damage".

3. Most of the classes are a bit deficient in teaching their tricks. Sure, when you hit the first move of a combo on dragoon the second move will highlight to indicate what you should do next. But the game wont teach you the difference between their two combos and how to most effectively balance them(one hits harder, the other has nice bonus effects - how much/when you alternate changes a bit as you level). Reading tooltips, looking at traits and action descriptions... doing a bit of in-game(or out of game) research basically is the key here. And even then, sometimes the importance of something can be hard to notice if it isnt pointed out by another player(for instance the ideal down-cycle of the black mage rotation changes completely the second you get the blizzard 3 spell, but its not at all obvious because its the opposite of what youve done until then and the tooltip you need to read to pick up on it lacks proper emphasis). So this point is kinda a wash.

4. Of those 4, black mage and bard are probly slightly easier to "get right" from the start, then dragoon, with summoner needing the most learning and management. The things that make the BLM easy to learn(easy rotation, long casting times) eventually make it harder to master and optimize. The utility that bard picks up somewhat complicates its patterns. Summoner is a bit finicky over pretty much all levels, but their combination of damage and utility(one of only 2 non-healers who can res) is nice. Dragoon gets a couple of the best buffs in the game, and the melee side of combat gives it a more visceral edge then the others you highlighted.

5. None of those classes do abysmal damage as long as you learn what to do(summoners should use dots and learn the keys to their pets, dragoons should finish combos and for the love of all that is holy keep your buffs up, bards want to use their dots procs and songs, black mage needs to take proper advantage of the elemental stances). If utility is the tiebreaker, bard has most, summoner and dragoon are basically tied, and black mage is lowest.

No idea if any of that helped. But i typed it.