If you have not already done so, you should look at the Coash message boards on ancestry.com and genealogy.com. The Coache family of your query is discussed extensively in that forum, Coash being the usual spelling of the surname in later U.S. records.
Other variant spellings I have come across include Coach, Cowache, Cowash, Corrache, Kovash, and Goache. The original emigrant for this line is Jean Coache, who (as per the 26 Apr. 1785 record of his marriage to Marie Duclos at La Prairie, QC) was the son of Francois Covadchy (spelled de Kovadchy by some researchers) and Catherine Bagnir of Pucy, Hungary.