If the device is starting up, displaying start up screen messages, and acting as a scope then it is somewhat unlikely that the DFU is corrupted. If it was badly corrupted then it wouldn’t start up at all, or display serial number etc.
Now it is possible part of it has got corrupted so that the USB disk emulation is not working, but it fairly difficult to corrupt the DFU by any normal activity. The main reason why this critical bit gets damaged is loading new firmware that has not been linked correctly so it overwrites the DFU memory area.
The two other main reasons why the DFU would not be recognised when plugging in would be a problem with the USB cable or an issue with the host PC DFU driver.
It is certainly worth trying a different USB cable and the other USB ports on the PC. Even if you think the cable is OK then it could be the battery charging current that flows down it causing a problem.
The host DFU driver can cause issues and people routinely struggle a bit on Macs, Linux and late Windows OS like Windows 8. You say you have tried Windows 7 which does not normally cause a problem but it might be worth trying running virtual Win XP if you can.
Before contemplating flashing a new DFU via the serial port method I would take a dump of your memory (see fldump utility near end of DSO Quad Boot loader corrupted thread. If you can find an owner of same hardware to do the same dump then you can compare the first 16KB which is the DFU. They should differ only in a few bytes where Licensing gets stored.