I was able to boot to download mode by holding the vol +/- key and then plugging in the USB cable but there is no movement from the computer it doesn't make a sound nor does it detect. I tried restarting the phone then I get a notification on windows that USB device not recognised!
When you say you changed the "USB flex cable", are you talking about the internal connector and board inside the phone? If so, perhaps the I777 connector board is actually a little different than the I9100 board and causing a compatibility issue.
The bug in UCKK6 related to wifi didn't affect everybody, but it definitely affected a lot of people (based upon reports here).