It could be possible that those 4 damaged bytes are the cause of the IS initialization failure.
Or an unrelated calibration difference, or flag recording the error ... (I have no preference for any of these theories)
From comparison with D10, which is similar generation and same ROM size, it looks like the normal "adjustment data" is at 0xFFFE0000 (EraseAdjustmentArea refers to a variable that has the address when running)
FFDF0000 seems to be the start of some block of data, before that it's all 0xFFFFFFFF (i.e. never used flash). I expect this address holds the same data for all firmware versions of the cam, and likely many other cams with the same ROM size.
FFDF0000 is referenced from ISCommunication.c in the firmware. Near the string "SIO_Int"
Our ixus110 100b, 101d and 101f dumps all have the same value (01 BC A9 02) at FFDF4AA4... which might lower the odds of it being a calibration value.
My d10 has a different value.
The next difference in poldy's dump is at FFFC0000, the there is only this one difference in the FFDF0000 block.
The ML cache lockdown hacks might offer a way to try "patching" this to the value found in other firmwares without modifying ROM.