I just got the Breathe of Life Seal and tested it out. It does in fact stack: Aura + Breathe of Life 3 + Breathe of Life 1 will net you 15 hp after Linde attacks and is adjacent to any allies.
This is what I was thinking, if only I had Linde before the Tempest trials I would've had a very reliable blue mage with me since Robin can't do much on his own.
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