Ok,
a lot of info here to work with,
As i understand it (please correct me if wrong) your light load tests are not blowing up mosfets? and the FETs are not getting hot in light load tests.
If this is the case it would be good to see the current waveform when the system is powering the full load. See if you can triggure on the hall effect and capture the peak current while the mostfet is still alive.
As a side note i have not often had success with home brew hall effect sensors so if yours is home made before you can trust the levels too much would be worth checking with a known current effectively calibrate it
Generally the schematic looks to have no issue, the design is solid, has adequate heat sinking (assuming the drain is soldered solidly to the terminal in that picture) the fly back diode is present assuming it hasn't blown up.
The gate resistor is generally a good idea but bare in mind the gate is electrically isolated from the drive and source so almost no current flows through the gate anyway so the gate resistor as you have found doesn't do very much. It will save your driving circuit if the mostfet starts conducting through the gate though.
As for the semiconductor temperature, your train of thought is a good one although not too relevant in your case considering what you have mentioned so far. If your case temperature was getting warm at all you would start to consider how well the semiconductor is bonded thermaly to the case, from the datasheet you can do calcs on the temperature rise of the semiconductor based on the thermal bonding of the case to your heat sink and the case temperature. But looks like you dont have to worry about this one until you start driving your motor under significant loads
If you system works (without blowing the FET) when driving the 1A motor and the precharge resistor the problem is likely to be the instantanious current limit of the FET 190A might be being violated your motor start current might be higher than 190A. If you are getting above 190A even in pullsed currents you should start worrying about this value. This datasheet value is related to the heat that the FET can dissipate through its case.
Can you tell me what your switching frequency is, you probably dont want to be above 100k Hz and you could likely be down as low as 1kHz. Although to avoid hearing your switching of the high currents you might want to be above the human audible frequency limit i think it is 26kHz
The mosfet direction looks good according to the datasheet
The oscillation is an interesting one, this will be developed by inductance and capacitance in your circuit which is inherent in parts and wires you likely havn't considered. Strange that it still happens under a resistive load did you try with a low resistance?, you might want to check your supply rails also do not have these oscillations. Also at these frequencies 12MHz you need to start considering ground loops so is your oscilloscope connected to a power outlet in which case you are likely to be connecting the 0V to your mains earth. Without knowing your countries mains arrangement and your oscilloscope construction i cant tell you for sure. If you are just running of a battery for your 12v this may not make a difference but if you have a mains connected power supply generating your 12v or a battery charger connected to the 12v this might affect your measurement.
A way to get rid of these oscillations (if not generated by your measurement methods) is to place a snubbing circuit (resistor and capacitor) wherever the oscillation is occuring however this is a last option. the best method to get rid of this is to find the source and fix it that way. So check with a low resistance load, try putting a capacitance on your gate drive, make sure the capacitance value is suited to the frequency of your 12MHZ as it will slow down your turn on time for switching if it is too large. Try things like moving the wires around while its switching and see if the frequency of ringing changes, the wiring inductance may be affecting it.
If you are correct in your idea that Vds ringing is affecting Vgs this could only happen if the source (s) is not coupled to 0v effectively. Check that the source of the mosfet has a low impedance connection to the 0v of the driving circuit
Its unfortunate the mosfet died again, you must be getting good at replacing them quickly now
. ESD is a good idea however the affect of ESD is very rarely observed in this way. I have walked up to a board accidentaly zapped it ( I felt the Zap) and the board works fine. ESD usually compromises the component silicon which then degrades over time. It is worth keeping an eye on ESD but i dont think its the cause of the dead FET.
It is interesting that when you removed the oscilloscope that the FET died, did it happen to occure right when you removed the oscilloscope 0v connection rather than the probe. As i mentioned above the 0v on the oscilloscope is connected to the mains earth effectively grounding your circuit. If you have FETs to spare and a bit of time might be worth repeating this, see if it is consistant, if it is this points directly to your problem well at least 1 problem. Check the connection of the 0v of the driving circuit, if the 0v on the drive is not coupled to the source of the mosfet then the drive could float higher than +20v and kill your FET and as soon as you try to measure the Vgs you would get what appears like OK levels but as soon as you remove the oscilloscope the FET could die.
You also mention you checked the temperature of the FET after it died, i recommend you put a temperature probe on the FET case so you can check the temperature when it is in a working state. alternatively put your finger on the case when it is working see if it gets warm under the full load condition. If you check the temp after it has died the temperature could dissipate within seconds or less considering you have such a large heat sink.
Well sorry i can not tell you the exact problem you have, its always hard to diagnose an issue without seeing the circuit and playing with it yourself
so good luck, look forward to seeing what else you find, just think how happy you will be when you finally work out the problem and you will have learnt a lot in the process