If I recall correctly, few people gave their opinion on that.
I am still not convinced that it is different from the other cases. The user will click or just hover over the Evil Queen and will be redirected to the right page, the Serum one.
It is not less susceptible to confusion than a viewer seeing 306 cast list and read Ursula. They might very well think "What, Ursula is Merrin Dungey and she is only in S4!". Then they will check and realize that is the other Ursula. And no need to add (Deity) in the cast list.
Again your exemple is COMPLETELY different, and out of context.
On 306 page, people will read "Yvette Nicole Brown as Ursula (Voice)", no "Merrin Dungey as Ursula". They won't just the character name, they will also read the actor name. So yeah, a little knowledge about OUAT will make them say "Oh, wait, this is not the Sea Witch, it is the Goddess because different actress". There is no need of adding a precision because it is not the same actress.
In the current case - and without the "(Serum)" - we would have two same credits "Lana Parilla as Evil Queen". And it is confusing. Especially if the reader do not know how we separate pages. Contrary to your exemple, in the current case there is NO way to know which Evil Queen we are talking about. So yeah, keeping "(Serum)" is absolutely necessary to make this difference.
On episode pages, we have to be as clear as possible at the first sight. On episode pages, people have to know immediatly what we are talking about. And, in the context of S6, "Lana Parilla as Evil Queen" is not clear enough. Also, many of our readers are using a mobile phone. Hovering is impossible. That's why we show the difference.
And that's what we already explained you.