Right, except I asked about their native language, which is not English 🙂

The correct answer is:

🙂

I suppose that’s a pretty solid indication that they learned it well 🙂

🙂

They both initially moved to England for university - him at 19 and her at 21.

That’s right, none of them is a native speaker of English - their English is learned via the standard combination of school, extracurricular courses, media exposure, and other typical means available in largely monolingual countries, and subsequently polished up abroad (at least two of them moved to Anglophone countries for university). Thus, I figured that the undercurrent of their native pronunciation should be sufficiently conspicuous to allow for making some educated guesses about their origin 🙂

„Then 3 hit me with surprise rhoticity and now I’m confused.” Hopefully at that point the challenge became entertaining 🙂

That’s an interesting and unexpected guess. It would seem to me that no. 3 in particular doesn’t sound even remotely Welsh, but perhaps absolutely wild guesses (to use your phrase) dispense with such stereotypical assumptions🙂