1.- Assuming, control is fast enough. Voltage spike comes from the wires. Best solution to protect the converter against sudden load disconnection?.
Seems like a thing that shouldn't be based on assumptions.
Voltage spike may come from the output inductor and di/dt
forced by the load. If the voltage exceeds VIN then it's an
inductive flyback. Whether this is the wires or the power
inductor wants checking. You ought to be able to figure
the value of each.
For getting a Cshunt value, try a simple SPICE simulation
with an inductor - capacitor - ipulse parallel circuit, the
ipulse initial value at your 260A, stepping to 0A in (say)
100nS, the inductor having the realistic values from the
converter and the capacitance initially using the converter
output filter values. You'll see a flyback voltage that the
C soaks up somewhat; more C should lower this and you
can work the series until you find happiness.
You may want to look at the disconnect scenario and see
if you can make it so hot always disconnects first, if
ground "comes apart" then maybe some of what you see
relates to measurement reference point. If you are worried
most about converter self-protection then make all of the
measurements relative to converter local ground plane.
Where the remote end swings to, may not matter as much
(and what is the load end's "perceived transient", relative
to its local ground, now distinct from converter ground)?