Richie said

"TY Richard Mellish for answering some of my questions."

Only personal opinions. Not worth more than anyone else's.

" In regard to number 2: Do you think that she is a mermaid in Child 42? Herd's copy is "Clerk Colvill and the Mermaid" and clearly she turns into a fish. "

All three of Child's versions of 42 call her either a "mermaid" or a "mermaiden". So in those versions she clearly is a mermaid of some kind, though apparently not the usual kind who live under the sea, as the location is "the wall o Stream", "the wells of Slane" or "Clyde's water".

" Comparing Child A and B to foreign analogues seems like comparing apples to oranges. In which British ballad did this reference water-sprite come from? "

Well, those are both fruit. And likewise the foreign analogues have analogous episodes, albeit with major differences in detail. It is to be expected that details would change as a story travels around the world.

As well as the Bayard article I've now read the others on your site concerning Child 85, including the Harbison Parker one which makes a plausible case for the change from an elf woman in the Scandinavian versions to a mermaid having arisen in Orkney or Shetland, where they apparently had no elves but did have selkies and possibly mermaids.

There is a quite separate instance of a mermaid in Clyde's Water. Joe Rae (Gutcher on here) has a story "The Carline Stane" in which a water kelpie in the river Clyde enjoys occasionally drowning young men and, to ensure a steady supply of them coming to bathe there despite the risk, changes an ordinary girl into a mermaid, who then sits on a rock in the river to entice them in. But unlike the one that Clerk Colville meets, who can shape-shift at will, the River Clyde one is stuck in her mermaid form until one young man kills the kelpie and kisses her. For the full story, buy the recording!
Musical Traditions Records MTCD313.

The Lady Alice versions start late in the story and give no hint as to the cause of the hero's death. In the George (etc) Collins versions his death follows very soon after his meeting with the fair maid, so presumably results from that meeting, but those versions don't explain how or why she causes his death, so the story still seems incomplete. If we take the events from Child 42 as part of the same story then it becomes quite clear that the mermaid causes his death, with the fact of his having married another woman as the motive, though the method is still not entirely clear.