Steve: In your opinion, Phil, what is the difference between celeusma and the known sing-outs (repeated phrase like 'Yeo heave' or '1,2,3') which are known to have preceded the chanties by centuries and were still in use alongside the chanties?

Difference would not be my word choice. Simple cadences; sing outs; fiddle/fife instrumentals; Spanish, French, Italian &c, and pre-19th century anything are not chanties. All are celeusma.