Gender locked race?

[Shippuu]

You forgot about the "young Elezen" stuff while Ravas and Veenas differ only with skin tones. Also, Feol Vieras don't look "drastically different" than the Ravas and Veenas, they are just shorter, especially their ears.
If the "young Elezen" like Vieras are the other sub race, and given how young Elezen are short compared to the adults, it's reasonable to think the other sub-race will be shorter Vieras -> Feol.

The young elezen comment was so vague and nonsensical that it could be used to explain all kinds of things. They were entirely talking about hairstyles for the most part so it could have been regarding a hairstyle. It could just be a reference to the vact that the viera legitimately look like elezen with bunny ears so a small one could absolutely look like an elezen. It could be reference to potential customization improvements to all races to give some mixed race options since those are a hugely requested feature. It could have been reference to something that never made it out of a concept phase.

People trying to interpret it as being a completely separate sub race option is a pretty big reach. It's not reasonable at all to think that when all the evidence points to the opposite. With 2 distinct clans being spelled out and their differences being the typical thing SE does to differentiate between sub clans.

[Kupocoubo]

The young elezen comment was so vague and nonsensical that it could be used to explain all kinds of things.

Yes, like a shorter sub-race, so we cannot just ruled out that option.