"Rosamund is born between 1860 and 1874, no other possible dates."
Exactly, so Rosamund could be older or younger, however Robert's page is very specific in saying that she is a year younger. I am simply saying that there is no substantiation for that level of specificity. Even if Robert were born as early as 1864, there was plenty of time between then and Violet's marriage in 1860 for Rosamund to have preceded him.
As for military training, (Robert was a noble not a royal) look at Prince William. He went to college, then to Sandhurst. Even in the 1880s, Robert pretty much needed to complete at Eton to go on to Oxford. He may have dropped out, but there is no indication that he did anything unusual, thus likely matriculated in the normal sequence.
Given that he served in the Boer war, one would think that he might have received training and served in the military for a couple years after leaving Oxford.