I completed the quest where you get the Skeleton Key and I don’t really want to continue the chain until I’ve done a lot of the other stuff in the game, since the key is so useful. Brynjolf said before that I would become leader of the guild after everything was over.
Can I become the leader anyway without finishing the Nightingales quest chain? Are there any bonuses for being the leader?
No, you cannot be named Guildmaster until you resolve your unfinished business with Nocturnal.
Benefits of being named Guild Master include a tribute chest full of (respawning!) loot, the Guild Master armor (an improved version of the Thieves Guild armor you were issued on joining, and a perk giving +25% Armor Rating when wearing a full set of Guild Armor.