If the model heals before it would get to the timing of the demise then yes. Remember that Demise(eternal) technically has an earlier timing than other demise abilities. Going to go through this step by step. Damage timing tells us to resolve each step completely for every model before moving to the next(I'm assuming this includes the substeps in step 6). So the mature damages at step 5, and now Joss goes through up until step 5 of damage timing. Since Joss is active he goes onto step 6.
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