No wonder I couldn't find Hope's previous marriage or birth! Ian must have gone out to Grand Cayman ahead of her, as he travelled out there in November 1957. Being American, she'd settle into the life there fairly well. Perhaps part of the reason for Ian and Margaret splitting up was because Margaret wasn't keen on the idea of living in Nigeria on a long-term basis. The heat and dust would take some getting used to, if you've lived in Scotland all your life!
I did at least find Hope's Probate records. I've also found one of Ian and Margaret's children: another Margaret, who married a Wotherspoon, ended up in Birmingham (Handsworth possibly) and married a second time to a Martin Annersley Line, son of a doctor. If she's the Margaret Mary LINES whose death I found in Solihull in 2016, she was born 29 April 1937.
I've also found Ian on the School and University Memorial Rolls 1914-18 - it looks like he went to Broughton School, Edinburgh, and joined the R.G.A. in 1914 as a Gunner.