"Valid variation"? Not in Latin or medical English. Words ending in -itis are feminine, so the adjective should be in feminine form. Compare *itis *osa and *itis *osum* on Onelook. Words ending in -derma are neuter, hence the -um ending with "xeroderma pigmentosum".
I don't know about that, sionnach. This whole series, as I said, takes place mostly in the Highlands or among Highlanders in the 1700s. Where Glaswegians, French, or people of other groups/accents appear, these "flavor" words are sprinkled throughout their speech. I find it helpful to identify who's talking at any given time, in addition to helping me hear the "music" of the accents.
I'd also argue this author isn't that lazy, as she uses Scots Gaelic words and phrases all over the place too. What exactly do you mean by "Highland cred"? Does one have to be brought up in the accent in order to write a character who speaks with it? That hardly seems fair.
Moreover, the list isn't a valid representation of her writing, since these bits I've been adding are the ones (naturally) that contain weird words, Scots slang, etc.
Oh. Yet again I'm reminded to explain that this series involves people (well, one person really) time-traveling from 1940s Britain to 1740s Scotland and France to 1940s-50s-60s Boston to 1760s Scotland, Indies, and Carolina backcountry.... That might explain some of the weirdnesses of language. Or maybe not.