My cabbages almost always have a few small slugs lurking in-between the loose outer leaves down near the stalk. They have never moved into a forming hard head or taken up shop inside there. If you have a huge case of slugs then perhaps a strong stream of water with a hose applied between the loose lower leaves would flush them out? Your salty water solution may work too as a follow up to flushing most of them out with a hose first. Salty water applied with a mister to the lower outer loose leaves only?

eta, as for harvesting, if the heads are hard to the touch (not spongy) then it may be time.

Those iron phosphate pellets work very well on all kinds of molluscs, IMPE, whether they're inside the leaves or not.