I think that you are over-worrying these roads. They are not treacherous. What's more important is to fill up on petrol and don't rush it. The A82 is main a road. I drove down as far as Campbeltown and, in fact, a bit further. I had my heart set on reaching the Mull of Kintyre, but I forgot to gas up and got cold feet when I realized the needle was.
If you look at street view for the A2 on Google maps you can see that it's a two lane road with marked edges and a centerline. It looks a bit like roads in Wisconsin to me. But maybe you're uneasy about driving in Wisconsin? You shouldn't be. The scariest thing in WI these days is the state government. (hah).
Now, if you want to continue south of Campbeltown as I wished to, the road does become single track. But I still would not want to call it treacherous. It's a single track road which means that when you have oncoming traffic, or if you are driving slower than the following traffic wants to drive, you need to stop when you come to a passing place so that the traffic can go around you. If the oncoming traffic reaches a passing place first, they will stop. If you are unsure about driving on single track roads, here is some information on them.
As for the paddle steamer, I would love to go on one of them as they are the historic way that people got around the Western Isles for years. i know it's a little bigger than the puffers that also serviced the islands, but the Waverley sure looks interesting to me! If you want to ride on a smaller steamboat, look into the Loch Katrine Steamboat.
Also, it looks like you can email the Tarbert Harbor Authority directly. email@example.com They may have some suggestions for you. I quite liked Tarbert.