1. I think there is no BR competitive guild as of yet, not sure about social guilds.
2. The bigger guilds typically have 30-50 on their peak/prime time. But most big guilds come and go as they are salary sustained. The usual guilds have around 10-20 regulars. Smaller solo guilds have around 1-3. As proper defense in this server is fairly difficult, solo guilds do pretty well at last min too.
3. Castle drops in this server are customized. The most common drop would be red boxes, which give supplies such as seeds and edps. The more valuable rare drops include: forsaken soldier card (headgear card which gives u immunity to stone), coloured emperium auras, eidilic wings, and imperial helms (colour depends on castle).
4. Monsters do not drop bloody branch, but you can purchase it from npc using event tokens. Hocus pocus is around but the mvp summoning ???? has been removed due to abuse. There is a mvp room that charges u for mvp summoning if you want another alternative.
You may want to raise further questions under general discussions section, you could probably receive more views there.