I've come into this thread rather late but can't resist adding my two-penn'orth.
As I read the position before 1917, the titles HH and Prince/Princess could go on ad infinitum. Ernst August, Duke of Brunswick (1887-1953), a great-great-grandson of George III, was treated as a Prince of Great Britain and Ireland in the consent under the Royal Marriages Act in 1913 and again in connection with the Titles Deprivation Act 1917. His eldest son, born in March 1914, was also officially declared to be a Prince of Great Britain and Ireland and an HH. Alasdair of Connaught is an anomaly. In my view he was HH Prince alasdair of Connaught from birth and remained so after 1917, as the Royal Warrant stated that it did not affect persons already living. There is some correspondence between Louis of Battenberg and the then Garter King of Arms, Sir Alfred Scott-Gatty, in 1915-16, discussing his position, but nobody actually got round to doing anything about him and he finished up simply as Earl of Macduff by courtesy.
I did publish an article on royal titles in an academic journal published electronically some time ago and I will post the link when I find it.
Ann