Hi. I am indeed still thinking through this mod. So here's a bit of a brain dump in response to yours.
Regarding coke, charcoal, and carbon. They are all indeed solid hydrocarbons with high carbon content. I think you are correct in that "carbon" can suffice. Since charcoal is made of wood in reality, carbon kinda sort represents charcoal. Since coke is made from coal, carbon also kinda sorta represents coke. The sulfuric acid part does seem to be Wube's own fictional spin, as sulfuric acid isn't actually used in the production of coke (though it can help with processing byproducts, but that's getting toward Pyanodon's level of detail....).
In reality, coke tends to be more energy dense than charcoal, so I think an early game charcoal item and late game carbon/coke item makes a lot of sense here.
Incidentally, coke doesn't need to be seen a strict upgrade over charcoal. For example, charcoal is more pure and so has uses in filtration that coke can't be used for. Could be interesting to add charcoal for some of the later game fluid recipes.
For space age, I need to decide if any Foundry building is unlockable pre-spaceflight. Or if e.g. Aluminum alloys that can be made in foundries only represent upgrades to improve recipes like LDS. Either way, given that Space Age has a foundry, maybe "kiln" is the way to go for the various solid hydrocarbon recipes (charcoal/carbon/coke/whatever). Maybe this current building would be called a "smeltery" and only be used for early alloys.... seems a tad complicated.
For 2.0 vanilla, I think I will leave things close to how they were for 1.1. I'm happy"swapping in" the kiln for the charcoal/coke recipes. I think allowing both charcoal and coke for steel would be fine if both exist. Maybe less coke is needed per steel (which would make some sense relative to reality), and so charcoal is mostly used for other things late game.
I understand your thoughts on assets as well. I like to match the factorio feel as much as possible in my mods. I'm not too concerned with the "kiln" footprint.
Overall I'm going to need to think on this a bit longer.