This is what I was getting at when I mentioned you didn't understand what you were doing with the voltage drop test. You understand how to do the test in general, but you don't understand what it's doing or how to use the answers you're getting.
The point is to
A - see if you have a problem in the circuit
B - if you do have a problem, narrow it down til you find the specific problem(s).
If you find a problem in the circuit you have to keep breaking the circuit down and testing parts of it until you find exactly where the problem is.
Basically, you check the whole circuit. Then you break it in half and check each half. Then break each half down into half again. ect ect.
You don't need to break it down exactly in half; use natural points at connectors and/or connections.
Underscore is right, you should have some voltage drop outside of the pump with the FPR in "normal" mode. That's the FPR sending power through the FP resistor.
Your best bet to check the circuit is get one of those jumpers I mentioned so the FPR circuit is always in "power" mode. This removes the resistor from the equation completely. Makes it easier.
Once you have the FPR bypassed you check the circuit and if the voltage drop is too high (say anything above .2v or so) then you need to start breaking the circuit into smaller sections and checking each smaller section individually. This will narrow down exactly where the voltage drop is so you know exactly what to replace or repair.
I'm going to try to break some of this down specifically for your car.
First off I wouldn't worry (yet) about the voltage drop with the FPR.
Let's start easy(er) and go with the voltage drop when you jump FP and B+.
You shouldn't have any voltage drop here. There are no relays. Just wires and connectors. It helps that the wire is all the same color.
It is possible there are several bad connections or problems that are combining here. So don't skip any section(s) if you find one that has a high voltage drop. Keep checking the rest. The only time you can skip is if you check a section and it has no voltage drop. If you find no voltage drop in a section there's no need to break the section down further.
Also, I'm going off the US wiring diagrams. JDM is similar, but there may be some differences.
I'm going to assume you know how to locate the wiring connectors as well as which pin is which in the connector. If not you need to learn. Only going to type so much.
As you are breaking this down into segments you're going to be checking connectors that join 2 different wiring harnesses together. These will be some of your "break" points. It doesn't matter which side of the connection you back probe, as long as you use the same side to for both checks. If you use one side of the connection to check one segment and the other side to check the other segment you won't be checking the center of the connection and the pins and crimps for the connection itself.
I know that's a clumsy explanation, I can't think of a better way to explain it.
Key on.
1 - First break point is going to be at the DIAG box. We're going to separate the B+ side and the FP side.
-One MM(multimeter) probe on the positive battery terminal, the other on B+ in the DIAG box. If it isn't basically zero you have a problem in the B+ wiring. Hopefully not because there's a bunch of wires and splices in this circuit. Goes all through the engine harness plus some chassis stuff in the front. Will be a PITA.
-One MM probe goes to the FP port in the DIAG box, the other goes to the blue/black wire at the FP(Fuel Pump) connector. Should be basically zero again, but I'm guessing it won't be.
2 - Now we're going to break the FP part into sections. Engine harness and (several) body harnesses.
-Leave one probe on FP in the DIAG box. Other one goes to the IG2 connector (It's one of the 2 large harness to harness connectors attached to the outside of the ECU. White? I know JDM has this.). Find the corresponding blue/black wire and check. This is one single wire in the engine harness.
-One probe on the FP connector, blue/black wire, other probe on the IG2 connector blue/black wire.
3 - Breaking the chassis harnesses up. This is where I don't know how close the US wiring is to the JDM. So you may have to get creative. Though I'm guessing it will be on the same side because the FP is on the L side of the tank, so the wiring probably runs down the L side of the car for both US and JDM.
-One probe in blue/black wire on the chassis side of IG2, other probe in blue/black wire of IF1. On US spec this is in the drivers kick panel (L side where the large relay/fuse blocks are). Don't know if it will stay on that side or be on your drivers side (RH). This is your dash harness. Could be problematic because this harness is where FP splices into the regular pump circuit. Possible the splice is corroded.
-One probe on blue/black IF1, other probe on blue/black FP.
4 - Breaking up the last 2 chassis harnesses.
-One probe on blue/black IF1, one probe on blue/black BP1, which should be at the rear quarter panel. Roughly the side of the rear seat. This should run from the kick panel along the rocker panel to the rear seat area. One single wire.
-One probe blue/blak BP1, one probe blue/black FP. One wire that should run along the L rear quarter panel area to the fuel tank.
Couple more notes.
We didn't check across FP and B+ with the jumper installed. It's possible that was a bad connection.
This also doesn't check the wiring inside the fuel tank, it only checks to the pump connector at the top.
If you go to check the actual FP circuit, with the relays and all, you only need to check from IF1 (connection at the kick panel) to the battery. The circuit separates at that splice in the dash harness I mentioned above.
Hopefully that helps some.
And wasn't too detailed.