As explained, the percentage thing is an average. With careful breeding and genetic testing (and a good number of purebreds to select from), you absolutely can breed the Coastal genes out of a line. When the parents produce offspring without Coastal genes, it doesnt matter how many times you breed the offspring (or its offspring, or its offsprings offspring, etc), unless you specifically add a Coastal, or a Coastal cross back into the line, the Coastal genes are gone and will not magically reappear (unless by some freaky luck you get an animal that has a genetic mutation identical to one or more Coastal genes - Which is unlikely in the extreme to occur).
Crossing Coastal Jags with Jungles would produce Coastal Jag Jungle crosses. As the Jag gene is Coastal in nature, do you really need the name Coastal there? Jungle Jag sounds better (its all in the marketing
). If you add another sub-species (remember, Coastals and Jungles are the same species), then you make up a new name for it.