I'm certainly not against more imaginative means of promoting and providing rolling stock better suited to the needs of the West Highland line, but I can't help but wonder how you feel that any of the loco hauled derivatives you've suggested would actually be advantageous over the current, or a comparable, multiple unit operation, given the limitations represented by the lengths of platforms and single line sections:
Platform lengths south of and inclusive of Crianlarich can accommodate a maximum of 6 x 23 metre carriages: To continue with the current splitting arrangement with single ended HSTs, even with power cars off the platforms at WHL stations, the maximum train length to Oban or Fort William respectively would be 1+3; a reduction in capacity from the 4-car class 156s that currently ply the Fort William route. There's also only one or two platforms, post lengthening, that will even be able to handle a 2+6 HST formation at Glasgow Queen Street (without fouling signals and points), and I believe these will be fully engaged handling Edinburgh - Glasgow traffic.
With 385s, using 3-car units you avoid the platform issue at Queen Street, but still lose capacity compared to the present day if splitting at Crianlarich, and if using 4-car units you're back up against the strictures of platform utilisation at Glasgow Q.S.. You're also adding an additional coupling operation at Helensburgh, and presumably would require another class 73 at Crianlarich waiting in the wings to take one or other of the portions forward, adding complexity and potential delay to the existing operation, even with autocouplers.
Of course you could run separate services to Oban and Fort William while maintaining current frequencies to both, but how much spare capacity is there on the single line sections between Craigendoran Junction and Crianlarich, and there is an increased (though probably fairly insignificant, in the grand scheme of things) crew cost and resource to consider.
I agree that there are more inventive ways to serve the needs of passengers on the West Highland line than with thirty year old 156s, or fettled class 158s: Personally I look forward to seeing a fleet of 4-car Stadler bi-modes (Shorter carriages than Super Sprinter stock) with panoramic windows introduced to the service once Cal-Mac take on the franchise.