Islay to Glencoe - Car Rentals

There isn't really an option.

People either drive their own cars to Kennacraig then park up and foot passenger over (mainly the locals, who then have a 2nd car on the island- cheaper and way more flexible to do it like that), take their own cars over or use the buses to Glasgow which connect into and out off every scheduled ferry. So there is zero demand.
There is local car hire at Campbeltown (no one- ways) and a couple of local companies at Oban (no one ways), then again at Fort William (no one-ways).

As on your earlier thread there are bus connections at Lochgilphead for Oban, also bus connections from Kennacraig to Oban, changing at Inveraray.

It's simply how people travel. It's how I'm doing it in a few weeks time. Train/bus from Glasgow via Dunoon/Portavadie (or maybe via Arran, to be determined) outward, direct via Loch Lomond to Glasgow on the way back, then using the Islay island bus service. For me personally the time saving of the expensive flight isn't worth it, no matter how scenic it is.

The lunchtime bus from Kennacraig provides a connection to Glencoe and Fort William at Tarbet (Loch Lomond).

Or, there is the twice weekly ferry from Port Askaig to Oban, or fly out to Glasgow in 45 minutes- morning and evening on weekdays, morning on Saturday and evening on Sunday.

You can fly to Glasgow on Loganair and rent at GLA, then drive to Glencoe. You can probably drop the car at the Islay airport without it becoming a one-way dropoff, since it's such a small place. Glencoe is actually a shorter drive from Glasgow than Kennecraig (if there were rental cars). EDIT: I missed Stuart mentioning flights.