"Silly question but does it actually stand for something? If it does, it should be spelled as QANTAS."

It was originally The Queensland and Northern Territory Aerial Service, though these days the company name is Qantas Airways Limited which was derived from abbreviating the words. It is perfectly correct to use mixed case since is not actually an abbreviation.

If it doesn't, then it should be spelled as Quantas following English grammar rules. Q is always preceded by a 'U', as per quantum, quality, quantity

Not necessarily, there are lots of words where the q is not SUCceeded by u . Qi, Qis and Qat all immediately spring to mind.

Dave