Scenery is looking great! As for the additional track (drawn in blue)...a couple thoughts. One, it would serve as a layover/staging area, but so would a straight stub-end track in the same place, without curving around to reconnect. An inbound train arrives in town from that track, drops off empty hoppers, then the loco is serviced, turned if necessary, and the outbound train made up with loaded hoppers collected before. It departs into that staging track and is rearranged between operating sessions by backing it out into the yard and fiddling it by hand, or reversing the process. But this doesn't accomplish your goal of seeing coal from mine to destination. Still, I don't see running it all the way around alongside the curve as giving you that much more operationally.
Two, the additional track (descending) would run very close to the ascending track already in place and you'd need a long retaining wall between them. As it stands now you may only need cribbing or retaining wall along beneath the mine.
I think it was mentioned before, but putting a coal dealer in town somewhere, perhaps in place of the spur along the yard tracks (with the layer of foam it'd be easy to install), or in addition to that spur if there's room, would be a good destination for a few of the hoppers. That tipple looks large enough to load coal in various grades...so that could add to the challenge of making up trains and delivering the right grade of coal to the dealer. Certain grades were better for home furnaces, for example.
Also, the factory/warehouse flat along the wall could take a load of coal. I'd put a grate or hole between the rails with coal down in it (assuming there'd be an auger down there to carry it into the building's boiler room).
AND, if there's room, there could be another spot for a hopper near the turntable. You don't need a coaling tower, necessarily, for many small terminals just loaded the coal right out of the hopper into the engine with either a conveyor or crane. Just a spur long enough to hold a hopper running parallel to the turntable lead.
Just a few ideas.
Galen