Was holystoning known on merchantmen?
Yes. ex.: the "Banks of Newfoundland" reference. And nearly every (I exaggerate, but...) text on merchant sailor's work repeats the "Philadelphia Catechism" bit about "...on the seventh day, holystone the deck and scrape the cable."

...a shanty was a song led by a non participant in the labour, with responses from the workers to time their efforts.
Not so, in very many cases. If the crew was large enough, the chanteyman just sat there during a capstan chantey. If not, he had to participate. In halyard chanteys, the chanteyman typically took his place at the "front of the line." He my have saved his energy, but he definitely worked along. Bunting chanteys: obviously the man doing the chanteying was up on the yard. etc etc

Who cares who calls them "shanties" or not? Even shore songs of stevedores were called "chanteys." Such a label, and its now-conventional categories weren't so established until after the fact. We're talking about nautical work songs, here -- close enough I reckon!

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A possible issue is that ever thought much about what they were doing enough that they were inspired to right it down. I believe Hugill may have mentioned this idea in trying to account for why chanteys weren't mentioned more in the 18th century literature (among other reasons), and I seem to recall someone at the Mystic Sea Music symposium this summer (sorry I forgot who!) making the same case for lack of historical info in maritime music. Likewise, the "sing-outs" -- in effect, "chanteys that are not like full songs" -- are only documented, in brief, in a few books, even though these same authors say that such singing was done whenever a hand was put on a rope.