others - such as the aforementioned "Ashokan Farwell" and "Da Slockit Leet" seem to spread around with very little melodic change

But quite a bit of change in the titles. (I don't know what the right pronunciation of "Ashokan" is, except that just about everybody in Scotland including me gets it wrong, and "leet" means something entirely different to "licht" or "light" - it's the list of candidates for a job or office).

What's a "moderately slow" strathspey, for example?

A lot of 18th century tunes were labelled "Slow when not Danced" - the composer expected a variety of tempi depending on the occasion. Most dance tunes have to be done at varied tempi unless they're invariably associated with a single dance.