Teddy saying "Benny, your alive!" at the climax was confusing for awhile bc Teddy says at the beginning that he's taking care of Benny because of his condition. So that would lead me to presume Teddy knew Benny was alive for at least most of the episode

Benny and his dad could share the same name but I don't think there's anything to suggest that it was the dad in the basement. They established pretty well that Benny had a skin issue, wore a mask (pics of him in the mask) and Teddy is his caretaker.

Working within the bounds of what was shown though, my thought is that Teddy has really been holding Benny captive all this time rather than being his caretaker. And maybe after realizing that Benny had tipped Darius off, he'd actually tried to kill Benny, hence all the blood on Benny's clothes at the end of the episode.

To take it a bit further, I'm pretty sure Teddy is actually the dad.

Teddy introduced the idea that he was Benny's brother but I don't think there was anything during the episode that showed that Benny had a brother. All we ever saw in the pics and videos were Benny, his dad (unnamed) and various famous people. Darius thought Teddy was Benny but clearly that wasn't the case.

When Darius almost walks in on Benny playing the piano in the bedroom but Teddy intercepts him, I see that as another domineering practice session like the one Teddy was wistfully watching later in the episode.

Who else but the dad would build a museum to honor other demanding/abusive/murderous fathers?

And who else but the dad would reject Darius's statement that "Not all great things come from great pain"? He's living the lavish life, eating soft-boiled ostrich eggs, because he pushed his son to the limit. Nah, the ends justified the means to him.