Is the opamp on the bottom an integrator?

Yes, specifically it's called a bias servo.

What for?

Setting the DC voltage at Q115 so that the amplified signal is AC centered on 0 V.

Q115 and Q114 form a Sziklai pair, essentially becoming a really nice single transistor. The Sziklai pair is used as an emitter follower whose load, Q116,117, is a constant current source. The whole circuit is a buffer (with high-pass filter): no voltage gain but lots of current gain. (AC gets through, but DC is blocked.)

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