For the Unborn Val'kyr the choice is pretty simple. H/H or B/B. A tiny increase to attack does not at all outweigh the massive health from the other breed.
I agree about the Fiendish Imp, as its max speed is 333, you can count on it being able to deliver its swap move against most other speedy pets, as the majority tops out at 325.
You are thinking too small with the Minfernal, if you want to focus on Explode, H/H is the only choice together with a pet using Sunlight. Minfernal have an advantage over Netherspace Abyssal in that it can bring Extra plating for defense. Spam Crush until near death while delaying it with Extra Plating, then BOOM!
A H/H Minfernal have 1725 hp, doing 690 damage with Explode
H/H Minfernal with Sunlight is 2587,5, doing 1035 damage with explode, which ignores damage reduction abilities. Ive tested it on elite pets with 50% damage reduction. they take the FULL amount!