Someone who can give you a comprehensive answer to these quiestions is Cornish Pixie. CP has done a great deal of research and written comprehensively on the subject so here's hoping Sharron pokes her head in.
You could start by reading an article CP wrote in
Cornish Studies (no 9), The making of a myth:Cornish Miners in the New World in the early Nineteenth Century Also in the Cornish Studies series:
Payton, P., ‘Cornish Emigration in Response to Changes in the International Copper Market in the 1860s’, Cornish Studies 3, 1995, pp. 60-82.
Payton, P., ‘”Reforming Thirties” and “Hungry Forties”: The Genesis of Cornwall’s Emigration Trade’, Cornish Studies 4, Exeter, 1996, pp. 107-127.
How did they travel. Well they didn't always end up at their original destination. The Wisconsin lead mines was one of the many parts of the US that attracted Cornish migrants (and many from the NE as well) and the Mineral Point Historical Society has the diary (his journey) of John Grundy online that might be of interest.
http://www.gundry.com/Diary/John_Gundrys_Diary.htm By the time the mining boom in Idaho started the Cornish Miners reputation was well established so they probably were head hunted to some extent. Many of them would already have been in the USA mining elsewhere so it would have been a mini gold rush.
I've no idea why they would have shipped the body back to Cornwall, or how come to that; in fact I find it rather surprising considering the many Cornish cemeteries around the world. Many miners did of course return to Cornwall prior to dying.
a rough guide.
MIGRATION OVER TIME
1816 Richard Trevithick in Peru, 1820's Major mining development in Latin America
1830 Cuba and USA recruit Cornish miners
1840 "The Hungry Forties" potato blight in Ireland & Cornwall
1840-1880 Australia mining development, 1849 California gold rush
1857 Australia, new South Wales gold rush, 1867 Collapse of the Cornish copper
1866-70 Diamonds found in South Africa, 1870-1900 Africa mining development
1871 Gold found in the Transvaal, 1872 Australia tin rush
1877 Silver found in Colorado 1880, India, Mysore gold mines run by Cornish miners
1886 South Africa, Rand gold fields, 1896 Alsaka, the Klondyke gold rush
Hope this helps a little.
The past is a foreign country: they do things differently there.