Now, how many commentaries are there of these Sutras? I'm only aware of the Sabara Bhashya. Are any of these commentaries translated in English and available free online?
Apart from Shabara, Prabhakara and Kumarila wrote commentaries on Purva-Mimamsa sutras.
Prabhakara school of Purva-Mimamsa translated by Ganganatha Jha.
Purva-Mimamsa in its sources by Gangantha Jha. It presented 3 commentaries side by side.
A HINDU CRITIQUE OF BUDDHIST EPISTEMOLOGY Kumarila on perception The “Determination of Perception” chapter of Kumarila Bhatta’s Slokavarttika Translation and commentary John Taber. I don't know much about this book. It might be useful, I am not sure.