> What decision-making process led to the idea of injecting human urine into a frog in the first place?

In the 1930s, two South African researchers, Hillel Shapiro and Harry Zwarenstein,[26] students of Lancelot Hogben at the University of Cape Town, discovered that the urine from pregnant women would induce oocyte production in X. laevis within 8–12 hours of injection.

-- from https://en.wikipedia.org/wiki/African_clawed_frog#Use_in_res...

The reaction is to Human chorionic gonadotropi - basically it's a marker which tells a human's body "You are pregnant, proceed accordingly". If you've got a womb and are in a reasonable age range this almost certainly means you're pregnant, if not it's a sign something went badly wrong. So, testing whether this marker is present means you know months earlier than you might otherwise.

Presumably the frog "Make eggs now" marker is different, but not different enough to ensure this effect doesn't happen, after all ordinarily frogs wouldn't be exposed to the urine of pregnant humans.

> Will Welsh clawed frogs be the next species that we thought were gone but had just become better hidden?

This isn't a rare species. It just wasn't in Wales and now it once again isn't in Wales. So that's like how Wales also does not have kangaroos. No danger the kangaroo goes extinct, there are lots and they're pretty competitive. But there aren't any in Wales (outside maybe a Zoo?) and so the ecosystem there does not have a kangaroo shaped niche.