You cannot get the Road to 70 buff except on JP worlds now. It's intended to get people to play on the lowest populated worlds but the entire game is becoming congested, despite it being a patch lull, because of the hype recently, so Preferred status was removed from everywhere except the JP region.

There is no drawback to switching to lancer/dragoon. Once you get to the level 15 main quests you reach Gridania, where you can go to the lancers guild and unlock lancer. It's a blue unlock quest and after doing it you open your armory chest and switch weapon.

You can level it up easily. Kill mobs until level 7, press H and do your hunting log and if you have unlocked guildhests, there is a first-time bonus of experience for each guildhest on each class. Along with your class quests, you should quickly reach level 15 and then you can do dungeons and the leveling/guildhest roulettes to further level the lancer.