Both forms would be correct, but the ing forms emphasize the fact that these things are happening now.

I cannot say what Eliot had in miond, but I suspect it comes from the month thing. . He's not saying at this moment in time, but during this month, lasting in time. The piece is about and emphasising a continuing period, during April, the whole month, not one day or one moment. So the breeding, mixing, stirring are all continuous. Just my uneducated impression.

See if you can get a response form someone who actually appreciates poetry. They may be able to give you a better perspective.