Learn French: Les cobayes coopèrent de plus en plus. - The guinea pigs cooperate more and more.

You could say Les cobayes coopèrent les uns avec les autres de plus en plus. Because coopérer already implies working together, it is not strictly necessary, but adding les uns avec les autres strongly emphasizes the reciprocal action between the guinea pigs.

Do you pronounce the final 's' in de plus en plus?

You pronounce the final 's' on the second plus, but usually not the first. It sounds like 'de plu en plusse'. Sometimes you might hear a liaison on the first plus before the vowel in en (sounding like 'de pluz en plusse'), but the final 's' on the very last plus is always pronounced to emphasize the growing amount.

Why is there a grave accent in coopèrent when the infinitive is coopérer?

Like many stem-changing verbs (such as espérer or préférer), the last é (acute accent) in the stem changes to è (grave accent) when the ending is silent. Since the -ent ending for the ils/elles form is silent, the vowel before it opens up, which is written with an è.

Does cobayes carry the same metaphorical meaning as 'guinea pigs' in English, meaning test subjects?

Yes, exactly! Just like in English, un cobaye can be the actual rodent, but it is very commonly used to refer to a person or animal used as a test subject in a scientific or social experiment.