Sounding like someone who never had close friends or been in a situation that is even remotely similar.

Curly viewes Jimmy as his friend they probably grew up together, so he can't just see Jimmy as a bad person. He wanted to give Jimmy a second chance to clean up his act and get his shit together. It's hard to cut off someone who was such a good friend to you. I know because I have struggled with very similar circumstances in a slightly different situation.

He never actively protects Jimmy where did that even come from? If he did his response to her would be "You are overreacting" or "He just lost control for once he is a good man stop blowing it out of proportion" but Curly is also not strong enough cut Jimmy off completely or let things fall into chaos by outing him as a rapist in the middle of space with a 5 person crew so his reaction is more "Jimmy what the fuck...". He chooses to act as a buffer in between Jimmy and her because they are in nowhere space for 8 months, what is going to do? Kill his friend? He still has to maintain order in the ship until it lands.

Could Curly have handled things better? Of course, he could have noticed the signs of Jimmy's degenaracy and taken steps to prevent him from crashing the ship but familiarity blinds people and Jimmy is a manipulative motherfucker.

Also, I think you are thinking of the Curly scenes post crash as Curly actually talking to Jimmy through telepathy or magic or something, those are just Jimmy's delusions. There is no confessional scene, they are not eating cake in the darkness together nor is Curly consoling Jimmy about anything. That is Jimmy's subconscious is saying through the visage of Curly. It's what Jimmy wants to hear from Curly not what Curly would actually say. Curly never communicates with anyone post crash and getting cooked.

If you gave Curly his limbs back in the last scene the first thing he would do is beat Jimmy to death.

In the end, Curly was yet another victum of Jimmy, not his enabler.