Usually the flow goes from the pump to the loader valve first. Then out the loader valve's power beyond port and to the accessory valves, and from them to the 3pt which is usually inside the transmission housing. With open center hydraulics the flow goes through each valve in series, never parallel.
If your loader valve has quick connects to the cylinders (to enable removing the loader) you could try testing at that point.
My understanding is that the loader valve's relief valve will trip if the pressure to the loader cylinders exceeds it's pop off value OR the pressure downstream exceeds it. I.e. the loader valve relief protects all the valves down stream from it. That means that if you have a relief at the loader valve and in the rear SCVs, and block the work ports on the SCV, then either or both reliefs would trip. That could result in what you're seeing.
Check if there is a relief pressure spec for the different valves. That's how is is on my Branson (per the shop manual) though the values are pretty close.