Is it permissible for an unmarried woman to pluck her upper lip (mustache), or is this considered altering Allah's creation? And does the silent emission of wind (passing gas) nullify wudu and invalidate prayer?

There is no harm for a woman in removing mustache or beard hair by shaving or plucking. Some jurists consider it commendable for her to do so, and some have explicitly stated its obligation. This is not considered changing the creation of Allah.