Any idea how to get fittings out of aluminum intake?

Silly question, Have you tried a pipe wrench ? The work thus far, penetrant and jarring from imparts, probably have helped. You would need the intake rigidly fastened down in some manner. Try a pipe wrench first on the hose nipple with as much helper on its handle as available. If it backs out drill a hole through hole the plug about the size of the hex and try the pipe wrench on it. There aint much of the plug to hook a pipe wrench on but if it turns at all you can get a better bite.