The east side does present some challenges for using a boat. I don't think there is a launch point anywhere near Kilmory and having visited the beach at Kilmory several times over many years would not want a boat moored off there or anywhere nearby... too exposed.
As Poly says it is nice any sheltered at the top of the loch at Achnamara which is still on the right side for you and you could enquire re a mooring there. What looks like a slipway on the Google maps up there is more like a small old pier head with a fair drop off at the end. You can put a SIB in next to it but not a RIB. But if you were mooring at Achnamara it would be worth the launch/retrieve at Tayvallich either end of the holiday.
I could bear the 20min run up the side of the loch from Kilmory to Achnamara to use the boat but the near hour long run all the way round to Tayvallich on a bad day meeting traffic on those slow roads would put you off using the RIB.
However if I were staying near Kilmory my first enquiry would be of the Ellary estate office who run the Castle Sween site and own most of the land across to Loch Caolisport. I've always found the owners, the Rogers family, and their staff very helpful if you ask respectfully. They are a business after all and for a price may well allow you to launch and moor at Castle Sween bay also leaving the trailer there perhaps.
The 10min run from Kilmory to Castle Sween would be an easy run.
The estate office is on 01880 770232 or email email@example.com