even though this is an isalnd install why dont you just a run normal radaitor 7-8th hose back to the garbage disposal through the crawl space?? You could evern run a 1/2 copper hard or soft line back to the disposal too.
hook it up solid or soft, either way it will work forever..
It can pump quite a ways., though I doubt you are going very far.
wether you would install an air gap or not is up to where you live.