Windows will show unknown usb device if the board usb port is not working right for ANY reason.
Could be enumeration, could be the wrong speed in the usb peripheral, could be something wrong in the code...
I would check that your multimeter measures exactly 22ohm from the usb connector contacts to the MCU corresponding pins, and do not measure short to VCC or GND.
And on D+ check that you measure 1.5k to Vcc.
Check that you are connecting the right mcu pins for D- and D+, perhaps at some point you design got the mcu pins connected to the wrong usb lines.
Are you using a 8Mhz xtal? if not, you need to make changes to your pll multiplier to make sure you run the MCU at 72Mhz, otherwise the usb peripheral will not work. It can work with a main clock of 48Mhz too, but you need to change the usb clock divider in the code too. Those are the only 2 valid speeds for the USB peripheral to get the right clock.
I dont remember if the bootloader by default works with a button active high or active low. Check what the mini schematic shows, whatever the mini has is the default. That can be changed though, I have done it for some boards that had the buttons the opposite way.
Also, I compiled one or 2 times with gcc 4.9, and the bootloader did not work. I never looked into it and just compiled with 4.8.3 and that worked fine.
To discard any compilation errors, I would upload one of the compiled bootloaders for generic boards from Roger's repo. We modified the bootloader to enabled the internal pull or pull down for the button, so if a pin is left floating, it will read like button not pressed, so you can use a bootloader compiled for a button in a different pin and leave the pin floating, and the bootloader should still work fine. And specially should be detected by windows, no matter what with the button.
By the way, you installed and tested your maple drivers with a maple board right? just to confirm the drivers actually work and load right.
Finally, about the 128kb, that is a nice free upgrade courtesy of ST. All the c8 we have checked so far had 128KB in reality, enjoy