السلام عليكم ورحمة الله وبركاته احبك فى الله احسن الله إليك (1) can I ask my mother to buy me something even if I don’t really need it? (2) do we trim the chin strap like we trim the mustache?

You can ask her, especially if she is responsible for you, but if you are able to provide for yourself then leaving the asking is better for you. As for the chin strap, it is from the beard, because the beard is what grows on the cheeks and the chin, and the instruction to trim reached the moustache alone.