Gobblecock
I understand your point of view, but I do not see this the same way. Erdrick won't save the "passive" slimes just because he's a nice guy, but because he wants to relocate a few (or most of them, if possible) to Grahferde. Saving the Queen Slime's life will make her more willing to cooperate (unlike Kek, who's already serving someone else), doubly so if she learns of the incoming Methuss' squads coming here. Moreover, since Kek and his boss don't know much about us yet (other than Mercant is not human), we could pretend to be the Angel from Methuss™ in order to put the blame on their kingdom, preparing a fun surprise for them when their retaliatory army arrives. That should allow us to weaken both forces with minimal effort, as Erd loves to do.
Besides, while I don't doubt these black slimes are way stronger than the average variety, they have been tampered with to prevent these from being captured, a fact which will most likely apply too when these slimes die and they self-destruct. Even though that makes them excellent shock troopers, at the same time they will be poor test-subjects, for not to mention Erdrick won't like putting his squishy bits inside an highly explosive jelly, especially if said slime identifies himself as a male. Don't you think so?
By now it can be safely assumed that slimes ARE sentient. They probably have some degree of autonomy when they're away from their Lord/Lady, but they get stronger and create a hive-mind when several ones merge (chances are this process can be reversed as well). Of course, that also means that killing or severely wounding the Slime Lord will deal a fatal blow to most, if not all of nearby slimes, which means that attacking both Lords is probably not really a good idea after all, so I'll modify my vote to just
A for the moment. Lastly, remember that one pink slime attacked of the rookie trainees on its own, so the multicolored species are not completely harmless, peace-loving hippies.
PS: By the way, considering that both Slime Lords are aware that Erdrick is inhuman, could we take this chance to remove his cloak and
flash show off his majestic "angelic features"? Combining that with his high charisma should be enough to intimidate them into submission, making this situation the first and last time Angelic Diplomacy is done. Unfortunately, knowing our luck, chances are they won't be impressed, crushing Mercant's ego even further.
PPS: Our companions sure level up very fast. I bet that's because they're considered monster-blooded (Aria most likely has a demonic ancestor somewhere within her bloodline, a fact which will both greatly interest and repulse Rin).