I don't know, but as a 14 year old, Mimi is limited to 3 wild cards in WTA/ITF tournaments until her birthday in October. She has used 1 in the earlier GB swing, so the W25 and W100 WCs she has won in the 18U use up her allowance. If she was to take two W25s in this swing, she wouldn't be able to take the W100 wild card. Grand Slam wild cards do not count towards age restricted wild card allowances so she will be able to take that even if she has used her 3 wild cards.
In reality the prize of W25 wild cards are academic. If you're good enough to win the 18U National then you would almost certainly be getting wild cards to the W25s anyway with the LTA policy that W25 wild cards should be for high potential under 21 players.
You're right. I looked back at my wild card post for reference, saw 1 and thought it was wild cards used, but it was wild cards available. I can't read my own own posts.
So Mimi has to choose between using the W25 and W100 wild card and can't use any additional adult wild cards she would get if she were to win the 16U.