We pickup a car in Glasgow and start our road trip by driving up to Oban for an overnight stay on our way to Mull.

Along the way, we plan to do a cruise on Loch Lomond and then drive through Inverary for lunch before continuing up on the A85 to Oban. However, I've been recommended to detour to Kilmartin and the reportedly more scenic A816 to Oban. Google Maps suggests this adds an extra hour of driving but to allow for slow traffic, I'd assume this would be closer to 90 minutes of extra driving, maybe a bit longer.

Is this detour realistic?

As an Aussie I've driven 8 hours direct (would not recommend!) so can manage a reasonable detour but thought I'd check as on paper it seems ok to me but I understand that traffic above the Central Belt is horrendously slow!

Are you flying into Glasgow from a flight from Aus? If so, driving up to Oban straight off the plane is a bad idea. What time of the day do you arrive in Glasgow. If early morning with rush hour traffic, then it's an even worse idea. Much better to stray overnight somewhere close the airport and set ff the following day. Kilmartin is famous for its prehistoric monuments but you won't see much with just a lunch stop or a scenery route. It merits a full day there and it will take more than an extra hour of driving in any case. Maybe include it on your return journey if returning to Glasgow.

I’ll be coming from Ireland or France (TBC) where I’ll spent about a week before flying to Scotland so will be well past the jet lag and used to the time zone by then. Will also only be hiring the car 2 days after arriving in Glasgow so will be a ok. I know a lot of people drive straight away (hence your comment) but it’s something I’d not be game to do myself.

That makes a big difference, but Kilmartin still merits at least a day or more if you're interested in the monuments. Otherwise I don't think I'd detour to get to Oban. The difference in the two approaches to the port is not that much, both scenic..

It's a big detour, worth it if you want to see the archaeological sites at Kilmartin - standing stones, stone circle, Dunadd, Carnasserie Castle - on route.Personally.it's not more scenic to me at all and I've driven either route many times. The mountain road above Inveraray as you come over to Loch Awe and Kilchurn Castle is stunning.You also have fine sights to see on that shorter more direct route too...the atmospheric ruin of Kilchurn as I mentioned, either a short stop for a photo or actually park and walk to the site.It's wonderful.Then beautiful St Conan's Kirk on Loch Awe, a very unusual and quite amazing little church.
Given you will have enjoyed the Loch Lomond boat trip, I think trying to fit in that extra, winding drive is a detour too far that day.