Not a very good idea. I feel like it would be absolutely detrimental and borderline unfun to get picked off in a 1v1 with nothing to fight back. The formula for support characters in the current game is already a solid way to balance damage and healing, in my opinion.
Absolutely adore this idea, especially the comparison to Alphonse. I can imagine this fitting into the game perfectly with the right design! The story and significance of the broken broadsword and his connection to the Hidden King are also written very well!