The current 08:23 split service was very popular with cyclists, with some having been turned away at the time. If they are going to recast the timetable I can see the Oban/Mallaig splits at Crianlarich being withdrawn in order to save time. This also means you can run a 153 on the end of both. The 8:23 also has some slack at Fort William, so maybe they could detach the 153 there and have it wait about for the next southbound 156, which should be the 16:05 from Mallaig, that has about 10 minutes at Fort William, so that should be enough time for them to reattach?
Though the issue would be pathing the Oban service in the morning if they abandoned the split. You can't run it out at 08:53 as that clashes with the 08:54 Alloa service. If you were to do, it then runs the risk of being caught behind the Airdrie - Balloch service at Dalmuir, unless you add some pathing time into it. This Balloch service also oddly appears to skip Dumbarton Central, so you might be able to easily run the Oban behind it. In terms of a pass, you could have the 06:03 Mallaig - Glasgow await the Oban service to arrive at Arrochar & Tarbet, which should only add a couple of minutes onto the time.
Alternatively, you could run it out at 08:36, but that gets you stuck directly behind the 08:33 Anniesland service.