> My impressions are that Child A and C are derived from B which is Herd and B is a translation from the German.

I agree that 42A and 42B are so close that either one derives from the other or both derive at a fairly short remove from a common source. But I'm not sure that the direction of influence is necessarily from 42B to 42A.

The original text of Child 42A, in William Tytler's Brown manuscript has:

11. Then out he drew his trusty blade,
An' thought wi' it to be her dead;
But she became a fish again,
And merrily sprang into the fleed.

"Dead" (= "death") is written with standard English spelling, but it should clearly be pronounced in Scots as "deid" (like the English word "deed" but with a shorter vowel), to rhyme with "fleed", which is the north-eastern pronunciation of "flood". In other words, whoever composed that verse, whether Anna Gordon or someone else, was thinking of the sound of the words as pronounced in the North East of Scotland. If not actually composed orally in performance as David Buchan would have argued, it at least looks to me like a version made to be sung.

The equivalent in Herd's 1769 text, Child 42B is:

9. Out then he drew his shining blade,
Thinking to stick her where she stood,
But she was vanishd to a fish,
And swam far off, a fair mermaid.

Here we can imagine "stood" pronounced either in a southern Scots way with a vowel like French "eu" or in a northern way as "steed". (I don't know how Herd, from Kincardineshire, would have pronounced it, and anyway, we don't know where he got the ballad from.) Either way it doesn't rhyme with the last line ending in "mermaid", but it would if this last line were replaced by something ending in "flood" as in 42B. This makes me think that at least this stanza in 42B may represent a "corruption" from something more like 42A.

Then there is verse 5 in 42B:

5 'Wash on, wash on, my bonny maid,
That wash sae clean your sark of silk;'
'And weel fa you, fair gentleman,
Your body whiter than the milk.'

The last two lines are intelligible like this all right, but "weel fa you" (= well befall you) is to me suspiciously similar in sound to "it's a' for you" in 42A, as if one might represent a mis-hearing of the other. And if I have to choose, I would say the corresponding lines in 42A:

"It's a' for you, ye gentle knight,
My skin is whiter than the milk."

make more sense in the context of the story, expressing the mermaid's seductive invitation to Clark Colven, to which he will recklessly respond in the "He's taen her by the milk-white hand" verse, which is missing from 42B, although something of this sort clearly should be there at the point where Child puts the line of asterisks.

(I presume Albert Friedman had a similar thought, as he replaces the two lines in his text of 42B with these from 42A.)

All a bit tentative, I admit, but here again, it looks to me as if Anna Gordon's 42A might well be truer than David Herd's 42B to the common source of them both. (Where that came from is another question...)