Need to Identify unknown components D53, CT7 and 22X
IC5 is definitely a USB TVS diode array. TI.com has some products of that category, and app notes that explain them.

Ct7 is a a Diode in reverse against ground; that screams "constant voltage or voltage limiting" all over, especially if it's indeed a Zener diode. You should be able to measure the breakdown voltage if it is &lt;5V within the circuit easily - neither the inputs of the LM358 nor the PIC are overly sensitive, especially when the rest of the circuit is unpowered.

Seing that IC4 has three connections to the MCU, it's probably something digital. But: the only pin not going to the MCU goes to In+ of the LM358's first opamp. Seeing that the opamp is configured in some kind of low-pass filter (capacitance in feedback) configuration, I "clearly" see a DAC with an anti-imaging output filter.

Wild guess: these inputs get sampled by the ADC of the PIC, and the DAC is used to bias/adjust the signal in some way.