A further thought about why QEII was given a tiara on her 18th birthday.
Although QEII was not declared the definitive heir to the throne (like a Prince of Wales), it was accepted that she was the Heiress Presumptive, and I suspect it was recognised (privately) that it was unlikely King George would have a male heir.
As we all know, under the British convention a royal heir is able to reign independently without a Regent once that person attains 18 years. (Think of the situation of Queen Victoria)
It was therefore a very significant milestone for the Heiress Presumptive to the crown, even if her father's health was not then a cause for concern.
I suspect that this, in part, explains why she was given a tiara for her 18th birthday and why Princess Margaret didn't get a similar present.
That Queen Elizabeth the Queen Mother gave QEII one of her tiaras rather than a new one having been commissioned might owe something to the austerities still imposed on the nation at the time, but it may also have been prompted by sentiment, so that QEII was given a gift which her father had given to her mother.