Can I become leader of the Thieves' Guild without finishing the main chain and is there any benefits from being the leader anyway?
Solution 1:
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.