If the darlington was failed shorted, turning the mosfet off wouldn't have an effect on anything.
By the way, a friend of mine sent me a link to Tesseract's comments about driving darlingtons. Here's what he said:
“What's the switching frequency? IIRC, you are using fairly old Darlington modules. Depending on the vintage, those were typically rated for 1.5kHz operation. Some VFDs of that time offered a "quiet mode" - 8khz, which is the default frequency today - but required a 25% or so derating.
Realize that Darlingtons require a proportional base drive circuit to achieve fast switching while minimizing Vce[sat]. Count on an overall beta of 100 (so a 500A controller will require 5A of base drive... and this is continuous, not impulse, current...).
The driver circuits used these days for IGBTs and MOSFETs will hard saturate a darlington which will lead to incredibly long turn-off times. This can be mitigated somewhat with a Baker Clamp, but then your driver power is going to shoot through the roof. Proportional drive really is best for Darlingtons (and, to be perfectly honest, is such a ***** to get working just right you might question whether you really got such a good deal on the Darlingtons in the first place ).
Here’s the link
Blackpanther_st's home brew DC motor controller. - Page 4 - DIY Electric Car Forums