I have had St Louis ribs require 8 hours. 8 might be too long for baby backs though. But, 6-7 would not be out of the ordinary.
I don't think the temps played much of a factor. I have smoked a lot of meat at your temps and lower without any adverse effects.
You start checking at 5 hours and if they need more time, shut the door and let them go for another 30 minutes, checking every 30 minutes until they are done to your liking.
I have been wanting to try potatoes, but haven't yet. But I think it is a great idea and I know a lot of people here do it and rave about them. I think most (but not all) put them above the ribs especially in the #1/#2 smokers as the potatoes do cause a heat sink especially with that many potatoes. With the extra space in the #3, you may get away with putting them below them ribs. Depending on the size of the taters I think I would nuke them for 5-6 minutes prior to smoking. I like my potatoes well cooked, so your time may vary.
I haven't done it, but I think heating the water pan juice is a great idea.
We'll see what the others have to say, but these are my thoughts.