Just want to point out that crossfingering C natural and half-holing C natural aren't mutually exclusive.
It's standard on the pipes to do both simultaneously, and likewise I do that on whistles.
It's not half-holing in the sense of fingering C# and having to bring the note down a semitone, but rather fingering C natural and "uncurling" the upperhand index finger to "shade" C, when C natural is a long note and/or emphasised note.
This in on a Bass A whistle, but you can clearly see (and hear) the "uncurling" motion of the upperhand index finger
https://www.youtube.com/watch?v=rJm6BQ-QxcgAlso I'm in the habit of blowing C natural a bit more softly, when necessary, to keep its pitch down.
That all being said I want whistles to play an in-tune C natural using oxx oox (the bottom hand can vary) and if crossfingered C natural is more than a tad sharp I might put tape on that hole.
I don't have any Sindts any more. I was able to compare a Sindt D to a Killarney D made a couple years ago, but not the current Killarney. My older and newer Killarneys play rather differently from each other.
(BTW on that video you can also see half-holing F natural (the note that would be F natural on a D whistle).