Well, he doesn't always completely turn off his pain sensors. mostly dulls them at most to make it easier to withstand.

Its not multiple powers. its all centered around inflicting or affecting one's pain as he can give or take it away. Considering pain is but something the mind creates to protect the body and signals which is the same for painful memories. The fluid is what his devil produces to do such effects and more so that it requires stamina to create said fluid so he can't go creating a huge amount of it without it tiring him out greatly. As for the memory thing. It requires the person to think of a strong, painful memory and it takes all his concentration to take that pain out which takes the memories involved with it as a side-effects thus it has to be done willingly.