The real danger comes from an advanced malware (2) and a 'bad' user habit (e.g. using same USB to transfer TX's back and forth).
You can never negate every attack vector. The goal should be to make the possibility of getting the private keys compromised as low as possible while still keeping the usability in mind.
You could set up a webcam for the offline pc and use that to load the tx rather than using a USB flash drive. A much more secure method this is, as you know for a fact that only the tx is being transferred.