Am I missing the Advance Single tickets for Edinburgh to Oban

They don't seem to exist, it is peak summer season so Scotrail probably don't see the need to reduce fares on trains they can fill anyway.

But there really isn't much difference in price. When they do exist (to the end of June) an Advance with railcard Glasgow to Oban is £16.15 each, whereas an Anytime ticket is £21.25, so only £5 difference.

You would book this as an Off Peak Single Edinburgh to Glasgow for £12.10, then an Off Peak Return Glasgow to Oban £31.90 (as opposed to an Anytime Return at £4 more)

AND an Advance Single Glasgow Central to London Euston. If you are on the 0850 from Oban on 27 July then book the 1335 from Central to Euston for £28.35 each, or if you want your £5 back, book the 1434 at £23.10 each.

The cheapest fare from Glasgow to Kings Cross (change Edinburgh) is £48.15, for a 1 hour longer journey.

That is your cheapest option.

PS- On this ticket type if you don't want to walk or take the Station Link bus from Glasgow Queen Street to Glasgow Central on 27 July you are allowed to get off the 0850 from Oban and get off at Dalmuir at 1139, then take the 1147 to Glasgow Central Low Level (this service runs every 12 minutes) arriving at 1213.
Then within the station go up to the High Level (Main Line) station.

Not something most guidebooks will tell you about.