Basically, A&E promised we’d get an answer to the question of why the UW looks like SB. Their reply: “YOUR QUESTIONS ARE POINTLESS” (aka the answer to every question about this show)
We actually are getting an answer in 516. According to overheard on set dialogue, Hades created the Underbrooke as a gift for Zelena. Something to do with how Zelena always wanted to be the one to cast the curse.
I wouldn’t be surprised if they have never planned for this and the only reason they will add an explanation is because fans started to ask them about it on twitter based on bts.