Then talk with your DM about substituting something else in place of the ability (ie just use the redemption version). A small change that could be significant would be allowing the spell effects in addition to the damage to be transferred to the paladin. This is what I believe is what they intended for the ability since the redemption aura specifically states it does not redirect any other effects of attacks but is notably lacking from the Crown ability. They are bodyguards for kings and the like, interposing their body in front of their lord.
I attempted to link the two abilities for quicker reference but I don't know if you can link class abilities. So...
Aura of the Guardian
Starting at 7th level, you can shield others from harm at the cost of your own health. When a creature within 10 feet of you takes damage, you can use your reaction to magically take that damage, instead of that creature taking it. This feature doesn’t transfer any other effects that might accompany the damage, and this damage can’t be reduced in any way.
At 18th level, the range of this aura increases to 30 feet.
Starting at 7th level, when a creature within 5 feet of you takes damage, you can use your reaction to magically substitute your own health for that of the target creature, causing that creature not to take the damage. Instead, you take the damage. This damage to you can’t be reduced or prevented in any way.
Again I personally feel that since it does not explicitly state that Divine Allegiance "doesn’t transfer any other effects that might accompany the damage" like Redemption, that it does transfer other effects ie knockback, secondary effects like set on fire et al.
"Where words fail, swords prevail. Where blood is spilled, my cup is filled" -Cartaphilus
"I have found the answer to the meaning of life. You ask me what the answer is? You already know what the answer to life is. You fear it more than the strike of a viper, the ravages of disease, the ire of a lover. The answer is always death. But death is a gentle mistress with a sweet embrace, and you owe her a debt of restitution. Life is not a gift, it is a loan."
If you were Physically throwing yourself in the way of the damage, then the last line of the description: "This damage to you can’t be reduced or prevented in any way." would seem out of place. First, when simply interposing yourself, you should maintain the benefits of a high AC (while wearing plate mail, at least). Also resistances and things.
So I'd lean towards the idea that it is a magical effect along the lines of "shield of catching" where projectiles, vaults of acid, and incoming melee damage, are all redirected towards you, or even more powerfully, like a living, breathing, inverse voodoo doll.
Nah, makes perfect sense to me to not be able to reduce or prevent it. I'm imagining the equivalent of a bodyguard leaping in front of a bullet meant for their client. Well, this would be leaping in front of an arrow meant for their leige, or a sword, or whatever, but to me, the inability to reduce or prevent the damage is what actually makes the image really click for me.
So, if I jump in front of an arrow or sword strike, plate mail ceases to be plate mail? My unaturally toughened skin (or naturally toughened, in the case of dragonborn.) Stops resisting whatever it is?