The two ground leads in the engine bay offer redundancy... they're very important, as you may know.
The 300 mOhm you're reading might not be accurate. To test low resistances accurately, you need to push a fairly high current across the device under test, then measure the resulting voltage with a voltage meter USING SEPARATE LEADS. For example, if you push 10 A across the lead and measure 3 V, then your measured resistance is 300 mOhm (3 V / 10 A = 300 mOhm). You must use separate leads for current injection; otherwise you'll be measuring the resistance across the current injection leads, too.
To clarify, set a constant current power supply to output 10 A, then touch one lead to each of the bolts attached to the cable. Now measure the voltage with your volt meter, but make sure you don't touch the current injection probes. This is called 4-wire resistance measurements.
The reason a standard resistance meter isn't going to accurately measure is that they only output 1 mA on the high end. With a 300 mOhm load, the meter is actually only reading 300 uV, which is a very small value... thermal emf and junction potential are all going to add considerable measurement error. Pushing more current overcomes those physical limits.
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The "HV supply" is the IMA motor, which is a Delta configuration, and thus has no neutral lead. You're thinking it's a Wye, which would have a neutral line.
Based on your symptoms, it sounds like you may in fact have weak ground leads. The reason they're so important is that the various low noise systems - including the IMA computer (MCM & BCM) aren't grounded to the chassis, but rather are connected to the engine's ground via dedicated leads in the loom. When the ground straps start failing, there's no solid ground path from the engine back to the battery and DCDC converter, which are both grounded directly to the chassis. This causes the engine ground to raise about the chassis ground, which cause logic ground reference mismatch, and then all hell breaks loose.