Starbuck_II
Explorer
Forgive me if I'm misinterpreting you're nitpick, but based on what I read in the PFRPG appendix only the highest DR applies. If the creature could have DR10/evil and DR5/good then you'd use DR10/evil.
I think it's also fine to say something in the abyssal bloodline prevents them from summoning "angels."
What gets people is sure DR 10/evil and DR 5/good: only DR 10 applies till they get pass DR /evil then DR 5 is still there.
So he has a 2 DRs and very hard to get pass both.
While you can buy a weapon that both evil and good (Holy and unholy); monsters don't get that ease.