The route you mention from just north of Cambeltown, via Loch Lussa back to Kirriegruer is on really good wide tracks with a fair bit of climbing. It’s probably not the most interesting section of riding and the views aren’t great by Kintyre standards. I’m not sure it’d be worth the effort with taxis etc. Luckily there are alternatives!
From a scenery point of view and less faff, if you start from Kirriegreur and pick up the Kintyre way just to the south and head across to Saddell Glen via the top end of Loch Lussa. It would be a good out and back and you could go all the way across, or just until you get great views of Arran and come back again. All the main tracks are clearly marked on the OS map:
http://www.bing.com/maps/#Y3A9NTEuNTAwMDAwfi0wLjEzMDAwMCZsdmw9NiZzdHk9ciZlbz0wJnE9UEEyOSUyMDZYQg==
There are a lot of good options from Carradale, again the OS map is your friends. There are some nice waymarked walking trails and fireroads to the north of Carradale which would be ideal for rides of any length. You could also make a very nice loop to south of Carradale by riding down the road to Ugadale and following the track to Loch Lussa , head up the side of the loch and return via the Kintyre way and Saddell Glen. This would have great views and make a nice non-techy ride. Again, all clearly marked on OS map.
Hope that’s useful