With too little damping on the LC filter, the gain peak will be higher and the phase will drop below zero near this frequency. It's not as bad as having no phase margin at crossover, but it is possible to get oscillation under some circumstances as there is significant gain at this frequency.
I'd probably just use a diode to clamp pin 1 to pin 15 if I was worried about overvoltage on that pin, but I see no reason why R12 would come open, even as a variable resistor if it's wired correctly with the resistive element wired like a fixed resistor and the wiper tied to one end. I suppose if you turned the output voltage down too quickly while it was running, the voltage at the pin might rise before the output capacitor discharges.
Gain at 1 Hz is not really significant in itself, its just the lowest frequency I plotted to give an indication of DC gain. The higher the DC loop gain, the better the DC regulation. 55dB gain here is adequate, but that all comes from your unusually high input voltage giving your plant high DC gain, only 7.5dB comes from the error amp. in a different circuit, this might result in low loop DC gain and unnecessarily poor regulation. With a capacitor blocking DC from the error amp's local feedback path, the DC gain of the error amp tends towards its open loop gain. It's irrelevant if the loop is unstable though.
I think the compensation you originally had came from a current mode controlled (CMC) power supply. The plant gain in CMC is very different as it by design approximates a current source into the output capacitor, so that gives high DC gain reducing the need for extra DC gain from the error amp. Of course, you can still include this extra capacitor in CMC loops for extra DC gain and type II compensation is common in CMC loops. In the ubiquitous TL431/optoisolator (which is a surrogate voltage error amp) in most power supplies you might see a capacitor between reference and cathode pins and that's what it's doing.
This is my understanding, and it's probably an oversimplification, but... The loop gain is a measure of the loops ability to perform its duty of regulation. DC gain is high, so you get good DC regulation; gain around 50-120 Hz is high, so noise from rectified mains is regulated away by negative feedback etc. However the PWM is essentially sampling the voltage at the switching frequency, so you don't want it trying to compensate for the switching noise as it's not sampling it fast enough for the PWM to deal with it. See
Nyquist Frequency. As well as this, frequencies which have negative phase margin and a gain of not far below 0dB might ring on stimulation even though they wont oscillate and the switching noise in a practical circuit can be quite a stimulus. For these reasons, you want it to ignore frequencies above half the switching frequency and crossover is normally set between 5-20% of switching frequency, often with the compensator's pole around half switching frequency.