Coplin
Apart from the isolated example of Alice Copling, buried at St Columb in 1632, the name Coplyn first appeared in the Falmouth district in the 1670s and 1680s with baptisms and marriages at Mabe, Budock and St Gluvias. Does this geography, near the Fal estuary, indicate that it had arrived by sea? Is it relevant that the other place in which this name was found in numbers in the late 1800s was Norfolk? Its meaning is anyone’s guess. Any suggestions?
Crothers
It’s been suggested that this surname may be from a crowder, or fiddler. This is unlikely. It appeared in Cornwall very late. Christian Crothers was baptised at Redruth in 1763 and the name then spread to neighbouring Illogan and by the end of the century west to St Hilary. It’s more likely to have been a local spelling of the name Carruthers, which supposedly has a Scottish origin.
Cunnack
Cunnack is unlikely to be a nickname from the Cornish word for clever – connek – as is sometimes claimed. This is because it was found nowhere in the Cornish-speaking parts of Cornwall in the 1500s or early 1600s. Yet it was present in Cornwall from early times.
The surname Connack or Connek was limited in the early 1500s to the Liskeard district in the east. It remained in that area until 1641 when a Nicholas Connock was buried at Truro. It then turned up further west at Madron in 1649. The first spelling of Cunnack appeared there, with Alice Cunnack, married at Madron in 1705. From that parish the surname Cunnack spread to St Ives by the 1730s. It’s more likely therefore that Cunnack is a local spelling of Connack or Connock, as the vowel /o/ often became /u/. But what’s the origin of Connock? The early concentrated distribution in south east Cornwall may imply an origin in a placename. Could it possibly be from a shortened version of Boconnoc, a few miles west of Liskeard?
Cornish studies resources 
Re Coplin, in your 2019 book p 108 you’ve noted a possible same-spelt-but-different-language origin nearby in mid Kerrier (Wendron) for Medlin. (In Wendron parish in 1622 Francis John was reported to have only slight knowledge of English, being almost a monoglot Cornish-speaker – cf A Symons, Baner Kernewek #96 May ’99 p6-9). If, locally, ‘middle-pool’ for Medlin, then perhaps ‘something-pool’ for Coplin. The 2nd element here, if so, would be the same as the 3rd element (Pen-an-linnow ‘top of the pools’) as has sometimes been suggested for Penaluna (though not in your book at p122, on clear place name grounds). On the other hand, being right on the coast then, could well be English. Don’t know though.
Just going to try also re Cunnack – maybe again a bifurcation of origins merging into 1 spelling? (gotta love them parish clerks) – but with a caveat (see on) – while Boconnoc d’seem very sensible as an origin given your name vs location findings for SE Cornwall.
In the texts is found (written down in1504) the play Bewnans Meriasek, ie the Camborne play, line1421 “ov bosy fecycyen connek” / “I’m a clever doctor” (spoken by the quack doctor himself treating the emperor) and line 1427 “why a proffse den connek” “you would prove a clever man” (spoken by the quack doctor’s sidekick Clericus Iankin, or clerk Jenkin, his line translated conservatively here as it might be a bit of a crude joke perhaps). Written down later, in 1611, in the play Creation of the World, lines 1405 & 1406, describing the attributes of Seth, Adam & Eve’s 3rd time lucky son, the word’s meaning is a bit of an abstract noun seemingly: “a skeans y fyth lenwys, hog a gonycke magata” “Of knowledge he’ll be filled, and of cunning as well”. Here’s the kicker though: both E Lhuyd and W Borlase in their glossaries (W Borlase published his in 1754; E Lhuyd wrote down his in the 1690s, both in west Penwith, Maddern way) have the same word as “Kudnik” – with a d before the n (this is called pre-occlusion, found all over the place in west Cornwall place names still). By 1649, 1705 and 1730, perhaps it might be expected that the name, if derived from the adjective, would be spelt this way too, with a d in there, instead of Cunnack with no d . Maybe, if that’s so, then this Madron name Cunnack may have not been Cornish language in origin, but instead imported from English. Maybe. Difficult to be sure on that.
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I had an idea Coplin could be of continental origin and checked this. It could well be a Jewish name, or the name of someone originating from the town of Koplin. That fits well with the record being near Falmouth.
Jewish (from Belarus): habitational name for someone from Kolpin or Kolpino, now in Belarus. German: habitational name from places called Kolpin or Kolpien, in eastern Germany.
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