Swords always look good, and the idea that Seraphim will fight with one sword and resurrect with the other seems quite realistic. You've convinced me about the championship: indeed, who else but a Seraphim? And the "mercy strike" ability could be implemented, but in the same way as it was for the Dungeon Lords: in addition to the main damage. That is, if there are wounded units in the stack, Seraphim first attacks with one sword, and then with the second, if not - only with the red one. Although, this is already an unrealistic fantasy.

Of course, it's realistic; in my opinion, this is the best explanation for the appearance of two swords. Well, Seraphim are combat angels, so it's quite logical that a Seraphim will take the place of the champion. And it could be done like this: if there are no wounded units in the stack, the Seraphim attacks with the red sword (normal attack), and if there are wounded units, then with the blue one (this activates the "mercy strike" ability). But if we judge realistically, this ability would be well-suited for the 3rd Tier Dis, but it will be difficult to implement in Heroes.