oscillation in overcurrent shutdown circuit
A partial answer to the cause of the oscillation is that the "thyristor like" unit does not remains in the on state after it has been fired (the same is true if it is replaced by a true thyristor). It may be interesting to understand why. Regarding a solution, I finally found that adding a feedback voltage divider to the sense + terminal does the trick; when this is done, the thyristor machinery become unnecessary, and this gives the following simpler circuit, which works well in spice simulations:


The principle is simply that the voltage comparator is maintained in the high state by the voltage divider whenever the FET is "off", while the sensed voltage is almost the same as without the feedback voltage divider whenever the FET is "on".