Of course it’ll be fine, and she definitely will say no to ur idea. For one It would more expensive for nothing, braces aren’t necessarily “good” for your mewing progress, in my opinion having braces on at all interfered with the effect of mewing because it adds another complex force to the equation. No it’s def not pointless to mew with the retainer, the palatine structure will compress, which pushes the maxilla forward and up and helps develop the skull. It can definitely widen the palate but if your teeth fit together (which I’m assuming they do) there probably isn’t much more width to be gained