'Percy's misdeeds' were surely already well known before Child was born. Child certainly was desperate to have the Percy Folio Manuscript before setting out on ESPB and indeed thanks to Furnivall he got that.

Child did bow to Grundtvig when it came to including most of Buchan's mediated material, but that would have complied with his initial stated intention anyway, which was to include all ballad material that MIGHT contain traditionary material, i.e., anything that looked like it contained the genuine ballad style. Buchan's 'eked out' contributions certainly did contain mostly pretty good ballad style, as he was mixing and matching from one ballad to another and putting in a good old sprinkling of commonplaces. Even his own pieces that Child included are much better imitations than those of the literati.

I don't think Grundtvig had any sway on Child over his sudden ceasing of criticising suspect versions, as Grundtvig was already dead by the time this happened.

Not sure I've noted any German I needed to understand.

Child's overriding desire was to get at the manuscripts as opposed to the published versions he used in ESB, and he was indeed very successful with this. Very little in the way of appropriate manuscripts has appeared since ESPB. He didn't get to see the later Buchan ms which was bought for the library after he died, but he didn't miss anything. He had been told by various correspondents in Scotland that there was nothing of any worth to add to what he had seen and copied in the BL ms. None of the Buchan mss contain anything like what we would call field notes as can be found in Motherwell's and Kinloch's Mss. The BL one is actually just a proof for the 2 volumes of 1828.