Apologies but unfortunately your question is not clear to me. may be you want to re-look at he stage design. Stages will have assignments or steps (utilities /smart shapes, etc) which either require an action by human or system. Channel merely represents the context, you can certainly expose / hide certain functionality for the user which is channel specific but if there is an assignment waiting for a user to take an action.
Channel is just mode of interaction, but it can't decide or control the case life cycle. Irrespective of the channel, the case will traverse through its stages in the predefined sequence with some conditional changes based on channel if any. We can't define case life cycle based on channels.
This is a classic and typical example for case management and adhoc case processing.
Rather than using stage based processing we need to break the whole process into sub cases and instantiate the process based on situation and conditions.
Read through case management feature of sub case instantiation manually and automatically
Case functionlity or Case life cycle can't be broken into subcases, like the flow and subflows, Subcases can only be created for case parallelism, or if there are any case hierarchy relationships exists. Stage based processing can't be replaced with subcases if it is part of the same case lifecycle, whereas we can reuse the steps which are already predefined in other stages or cases.