A little update to this.
The original flash chip got soldered in to the donor device and all is working as intended.
As discussed above - the flash devices, even though from different vendors, were compatible my case. In ABL's case - the flash devices were incompatible - hence a straight swap was not possible/working.
For anyone in the future, who wants to try their luck in doing this, I suggest you look for a donor that has a similar configuration flash device. The challenge might be to decode the part specs, but google always helps.
In the process a heatsink was added to the MCU as a general precaution as I have read that they sometimes fail due to overheating.
If anybody is interested in seeing pictures, give me a shout.
Thanks a lot to everyone involved!