So! To follow up on this, it looks like El isn't finding out about Winnie's mirror ICly (at least anytime soon), so based on the information Raven/Gilbert gave about the body in the lake emanating a similar aura, Eleven fully intends to bring a thawing undead to the lake. He doesn't plan to force the undead into the water or disturb the body in there, just see if anything happens/the undead's behavior or appearance changes at all in the vicinity of it.

He'll have brought Hendrik and Arche with him as back-up in case something terrible does happen.