One answer to the discrepancies is that each leg has a choice of routes. Or maybe the books are not very good! Make sure the car has a full tank of fuel when you collect it. The plan looks sensible (actually some of it is in an itinery I have planned for September!!)
Day 1. Airport - Oban assuming it isn't peak commuting time around Glasgow. Both routes cross the river on the Erskine Bridge and head alongside Loch Lomond, they would both take about 2.5 hours without breaks but I doubt very much whether you could manage not to stop somewhere! Beyond the village of Tarbet you can go either way.
Left takes you down to sea level and the village of Inverary which is worth a walk around. Then via A819 onwards. Note village and parking is beyond the A819 intersection so don't turn if you want to visit, carry on and come back afterwards! A819 is a nice drive and if you pay attention about 14 miles after Inverary there is a stunning view of Kilchurn Castle on the left (subject to weather). At the end of the A819 turn left onto A85 (everything is well signposted).
Right at Tarbet takes you north into higher ground with views into some serious terrain, although the road is a good one so nothing to worry about. After the village of Tyndrum you turn onto the A85. The villages on this route are not a pretty as Inverary but the scenery is good.
After the A85 passes the end of the A819 both routes are back together and just half half a mile later you should pull off into a small road on the left. This takes you to the parking area for Kilchurn Castle. It's all free of charge, you've come all this way, it would be insane not to visit (unless it's raining hard in which case teh access may be flooded!)
after that you just go back to the main road, turn left and keep going all the way to Oban. As I said, 2.5 hours driving but allow 4 - 5 hours with stops!
Day 2. You are looking at at least 4 hours driving plus some added time. Leave Oban the way you entered and just before Connel Ferry you turn RIGHT (sounds wrong, on a basic map it looks like a left). Take A828 to Fort William. You climb a little to get onto the bridge. The road to Fort William spends much of the distance following the shore line. At Ballachulish cheap maps suggest you turn left, but you actually go beneath a large bridge, continue a litle way and then meet the A82, then turn right and climb up onto the bridge. Eventually you reach Fort William which has any facilties you may be needing but isn't that pretty. At various points on the right are views of Ben Nevis which is the highest mountain in UK.
After Fort William there are 2 choices. Just before the Ben Nevis distillery (and visitor centre)
there is the intersection with the A830 to Mallaig. This road runs via Glenfinnan with it's famous (thanks to Harry Potter) viaduct plus the monument to Bonnie Prince Charlie. The road mostly follows the railway where you may see the Jacobite steam train http://www.westcoastrailways.co.uk/jacobite/jacobite-steam-train-details.cfm
after Arisaig there is a road on the left signposted Back of Keppoch which runs near to teh sea with better views. This is the area where movie Local Hero was filmed (Burt Lancaster's oil tycoon fell in love with beach etc..), the road loops around and comes back to the main road.
From Mallaig you need to take a ferry, which can involve a wait of around an hour depending on how lucky you are with arrival time. https://www.calmac.co.uk/mallaig-armadale-skye-ferry-summer-timetable