The Queen is the Fountain of all Honours. The decision will rest with her and her alone.
The law is that the wife of a British Peer takes his title, style and is ranked along side her husband. Royalty operates under the same system. While Camilla would be legally entitled to be HRH The Princess of Wales (HRH The Duchess of Rothesay in Scotland) I doubt this would be the case. Remember Wallis Simpson she was by all rights fully entitled to be HRH The Duchess of Windsor (and also Princess of the United Kingdom of Great Britian and Northern Ireland), however, this is not what happened. The King used his position as founs honourum to alter her status and title.
I would also point out that neither Sarah, Duchess of York nor Diana, Princess of Wales retained any title after their divorces. Those were the common styling for the divorced wives of peers. They are no longer titles but rather mere surnames that carry no rank or position.
Sarah is once again a commoner. Diana, however, remained a member of the royal family (per her divorce agreement) with considerable concessions such as use of the Royal Aircraft, use of the state rooms of ST. James Palace and her apartment in KP. Sarah has non of these concessions and her divorce made no provisions for her to remain a member of the royal family. Though, Diana held no rank despite still being a member of the RF. Diana started her life as The Hon. Diana Spencer then became The Lady Diana Spencer - HRH The Princess of Wales - and remains forever Diana, Princess of Wales.
I suspect Camilla would be known as something unrelated to any of the titles Charles currently holds.
I would also point out that Camilla wouldn't be HRH Camilla, Princess of Wales, Duchess of Cornwall...........it would be HRH The Princess of Wales, The Duchess of cornwall. The absence of the 'the' marks a gigantic change in status. Charles isn't HRH Prince of Wales he is HRH The Prince of Wales. Diana became Diana, Princess of Wales because she was no longer The Princess of Wales nor even a Princess of Wales it was a mere surname. I suppose more accurately a style, if you will. If she were Camilla, Princess of Wales it would imply that she were the ex-wife of a/the Prince of Wales.
What amuses me is how Sarah constantly calls herself The Duchess of York or Sarah, The Duchess of York. Which makes me laugh...that would imply that she were The Duchess of York in her own right or was still married to Andrew. And, she should know better. I remember vividly Sarah saying her daughters were HRH 'The' Princesses Beatrice and Eugenie of York on David Letterman. All I could think was that she knows full well that 'The' is only for the children of a Monarch and that her daughters were HRH Princess with no 'The.'
If William and Harry marry during EIIR reign their wives would become HRH Princess William of Wales and HRH Princess Henry of Wales. Though, I suspect that they will be granted other titles on their marriages. If they marry after their father becomes King their wives will be HRH The Princess William (until he is invested as The Prince of Wales) and HRH The Princess Henry. I don't think a British Prince has remained simpy "Prince Name" after marriage for any of the generations I can think of.
HM may even modernize and allow wives to use their own names in theire titles before the deaths of their spouses. Keeping that in mind, I also think that LP will be issued to allow their children to have the status of HRH Prince/ss. As it stands now, only the firstborn of William will be HRH Prince/ss his other children would be Lord/Lady and commoners. The children of Harry would be Lord/Lady and also commoners. I don't think this will happen as they would start out as commoners and end up as royals eventually when Charles became King. Besides, it would hardly be fair for Will to have one child be HRH Prince Baby1 and then have The Lady Baby2.
Perhaps, the old Windsor Dukedom will be revived for Cammy.