OK, finally got it working (it = MCU). Now I have an “evaluation board” with USB, SPI flash and buttons. Even the signal generator is back to life. The screen however was not lucky as well - only backlighting works that definitely doesn’t help in hardware debugging.
To get the MCU I had to recompile ST’s USB DFU example with some fixes in it. However the DFU mode button (PD9) doesn’t work and I still don’t know how to fix that. I’d appreciate if anyone could attach the original DFU bootloader to this discussion - would save me a lot of time in the first place but may yet definitely help.
I’ll see if there are displays out here for some acceptable price - however I don’t want to spend a lot more on $89 device (already spent some $11 for two MCUs). Specs on the LCD would also help.
At least it was a good way to spend a few days and learn a lot more about bootloaders and firmware flashing while I wait for my new DSO Quad 
))