As far as I know, Wood Release can only be achieved through transplantation of Hashirama's cells. Was Moegi an experiment or something at any point of time? Or did she just 'happen' to get it through sheer genetic coincidence? What do you think? Any theories?
We don't know. We can assume it comes from the Senju clan. But it's highly unlikely. Although... Her name "モエギ" can be translated as "yellowish green - 萌葱", "Young trees - 萌木" or "Burning tree - 燃木". It all goes to Wood Release.
It's my option:
Katasuke used Wood Release against Momoshiki with Shinobi Gauntlet in the novelisation of the eleventh film. Moegi's age, Konohamaru repeatedly used Shinobi Gauntlet. Perhaps Moegi also did, hence her Wood Release technique.
If I recall correctly, Wood release is not a KKG of the Senju clan but rather just Hashirama's unique KKG. It's stated on the page that he neither inherited it from those before him, nor did it pass on to anyone else. I've always understood KKG as not just abilities that either some or all members of a clan have, but also just personal abilities held by an individual and that's one of the reasons Wood Release was so valuable among other things. It hasn't been explained yet where Moegi got WR from because we know so little about her. Personally, I think she's supposed to be the fill in character for Yamato, but in the end who knows.
Well not just unique abilities, otherwise every technique used by only one person would be a KKG, it's explained by Kakashi pretty damn early that KKG have a genetic component, so even if you have an ability like the Sharingan, unless you possess that genetic component as well, you won't be able to duplicate those abilities.
Just to be clear, I'm not saying that WR is a Senju KKG, it's still only Hashirama who through whatever genetic lottery was the only one to unlock the ability naturally, but this doesn't mean that it would be impossible for his descendents or those related to him to gain the same ability.
Sorry if I wasn't clear. I wasn't denying the genetic component of WR, because obviously it is since its a KKG. What I was trying to point out is sometimes a KKG is not just a genetic ability granted by those of a specific clan, but it can also be a genetic ability held by a single person within a clan and I believe WR falls in this category.
Why aren't you considering using Shinobigote type ninja tools? Given that the Tono Katasuke lab studied Wood Release along with the rest of the elements, I'm more than confident that Wood Release Moegi is a Shinobigote result.