Do they still do this when the Prince in question has been granted a peerage in his own right? For instance, Edward is now the Earl of Wessex, and is expected to become Duke of Edinburgh after Prince Philip dies. So his wife is Lady Wessex or the Countess of Wessex.
Generally the commoner wife of (a) an actual Peer or (b) or his eldest son, or (c) that individual’s eldest son* uses the feminine version of her husband’s rank, and his title (the name he uses after “Lord” or “Duke of”). Presumably this extends to great-grandsons as well, as long as the supply of titles doesn’t give out. In the case of (b) or (c), and so on, husband and wife are both commoners; the titles are just a courtesy. They can run for election to the House of Commons and do anything else that commoners are allowed to do.
OTOH, in the case of younger sons using the honorific Lord John Smith, their wives do become Lady John Smith, and I assume the usage you describe for younger Princes works the same way, absent any Peerages they may have been given in their own right.
Certainly Viscounts in their own right are Peers. Anyone from a Baron on up is a Peer, and Viscount is the next rank up from Baron. Be that as it may, I’m not sure any actual Viscounts exist; I recall reading somewhere that at present, all Viscountships are held by higher ranking Peers, with the title of Viscount being used as courtesy titles by their eldest male line descendants. Technically speaking, therefore, these Viscounts are indeed not Peers.
(Yeesh, why do I know this stuff??)
I’m not either, but I’ve picked up nearly all of this information from P.G. Wodehouse stories, which AFAIK describe the styles and usages accurately, with the caveat that things are probably less formal now; for example today a friend of Clarence, the Earl of Emsworth, might well call him Clarence, while in the Wodehouse stories that was done only by his brothers and sisters.
(I mean, you know, for a king.) Or maybe I’m taking too much from