This has the same ground on both sides? Can you show how this is inserted into the FB loop of your DUT/system? Think there is some context missing that will help.
Reason you might have the resistors (R3/R4) either side is to reduce the effect (and phase shift) that the transformer introduces to the local FB. Try and imagine what it would look like without them. Same reason that the 100ohm (R5) is many times bigger than R4, it reduces the effect that load variation will have on the operation of the circuit.
Ignore that - just had a look at your linked sources. The OPAMP is the DUT.
You could reduce the 100R if your signal generator has the drive; 100R will have a protective function to your signal generator.
R3 is the part you are injecting across, it is low so that you don't affect normal operation of the circuit - Dr. Ray Riddley has an excellent set of videos that show this.