The buses to Acharacle operated by Shiel Buses, nbrs 502 & 506 , are principally school buses. They go from Acharacle in the morning to take children to school in Fort William and bring them back in the afternoon.
You can get to Acharacle by late afternoon on Wednesday but you may find a problem finding accommodation as the last time I checked the Loch Shiel Hotel is full on Wednesday night.
An option is to take that same bus on to Salen (it is a request stop) and get a B&B/ Hotel in Salen. You would then be able to get a return bus at 08:45 to get to Acharacle at 09:00 on Thursday morning.

Yes, hit the same problem. Sort of solved by using the Wednesday 00:30 sleeper from Preston to Fort William arriving 10am-ish then hang around for the afternoon bus to Acharacle for a Thursday start. The only loss is an undisturbed night's sleep but the upside is rolling through the highlands in the early morning.