If I had to guess, The Monero coal chute probably pre-dated the use of 2-8-2's (and the coal towers in Chama, Durango and Gunnison) on the line, and that some hard working Consol or Ten wheeler would need coal somewhere between Chama and Durango on certain occasions. Given the proximity of the mine, and the ability to have gravity feed in and out of the chute, it was probably a great choice even if not centrally located between Chama and Durango. It had to beat shoveling out of a gondola or off a platform that had been preloaded with a shovel, or even out of a coal trestle with pockets that required some one to shovel it out of the gondola above.
Is it possible the tower could have been provided by one of the mines? It would be a good PR move with the company doing your shipping.
As for only loading locomotives, that makes a lot of sense. If the coal tipple there was to load commercial loads as well, there would be a need to weigh what went in a locomotive vs a gondola. Having a separate loading area for commercial coal makes accounting a lot easier. Anything that went in the trackside tipple could be billed to the RR. Anything loaded on the siding would have a car number to be matched with a weight somewhere.