常用手往外掰她的上牙,平时没事时用舌头顶上牙. That will make the upper teeth stick out. That doesn't look good.
Using the mask is different way. That will force the whole upper jaw move forward. Won't change the shape of teeth.
I personally think the "mask" way is the right way. I would strongly suggest you to google how the mask way works (I did not do so).
Hopefully you all understand what I mean.