For example unloading coal from a cart directly with an unloader will input into the finished slot of a coke oven. Loaders also have an issue not inputting their buffer.

Ah indeed.

Heh, appears that I swapped the functions for extraction and insertion checks.

Loaders also have an issue not inputting their buffer.

Explain.

Any items in the buffer will not transfer from a loader into a chest cart.

Also I should probably mention its only items loaders

Ah, I think that's part of the same problem.