R4 is a 10k to ground on the output of the DAC going to the first voltage follower. Why?
They might have included R4 as a safety in case the output of the DAC went high impedance so the load current does not run away. But if that was the case, then a resistor should have been placed from the gate to the source of the MOSFET as well.
What is the purpose of U7A? Just to be able to provide more current to the voltage divider for U7B? (mcp4725 DAC can only supply 200uA)
That DAC should not have any trouble driving a 12k voltage divider so I do not know why U7A and R4 were included. They selected a quad operational amplifier so maybe they just wanted to put the extra operational amplifier to use.
The voltage divider is used to match the full output range of the DAC to the more limited range of the current shunt yielding the maximum resolution.
The purpose of U7B is to set the correct current across the shunt(just below the fet not shown). But what is the purpose of the passives I've highlighed, R7, R8, and C19? Is R7 to help match input bias current from R6? I see that C19 and R8+R7 make an low-pass filter, is this just for output stability?
The input bias current of the AD8630 is too low to bother matching the source resistance and the value of R7 would need to be 1.6k. R8 and C19 are part of the frequency compensation to handle the delay through Q2 because of its high capacitance. They require R7 to work against for them to have reasonable values; without R7, they would have to be much lower impedance to work against the shunt resistance and U7B would not be able to drive them.
The pole and zero frequency compensation indicates an effort to make well tuned design.