How did you end up fixing the issue? And the 'must have' to get into Download Mode...
I had the same issue but was able to either get into TWRP or get into Download mode.
Remember always connect to the PC when trying to enter download mode. It may seem rudimentary, but I have walked people through some serious stuff and the underlying problem was that there they unplugged the phone and there was no USB connection.
I know your phone was plugged in due to all the partitions coming up, but sometimes people unplug and forget.
How did you solve this problem may I ask?
By power + volume down, release power + volume down to get into TWRP or Factory Data Mode? Or the Ubuntu approach?