Just employ logic.
Using a USB memory that was frequently used on a Trojan-infected PC on an uninfected offline PC
Obviously connecting an infected USB stick to a clean device may result in that device becoming infected. Whether or not it is offline does not really matter here.
remote control malware
Your offline PC is infected. This is not your PC any longer. However, the PC is offline, so the malware cannot receive instructions from its owners over the network. It may, however, have already come with its own set of instructions, in which case it will do whatever it was programmed to do.
bluetooth is present but disabled in the device manager)
Then the malware re-enables bluetooth and might listen to further instructions over bluetooth, however this obviously requires that whatever is sending these instructions must be in close proximity to your PC.
The hacker is an ATP hacker.
Seriously? You worry about being specifically targeted by APT? And you ask questions on this site?
- It is most likely that your worries are unfounded and no APT is targeting you.
- If, however, you are indeed targeted by APT then you have a big BIG problem and resources such as this site are not going to help you. Basically you are screwed unless you can hire comparably badass defenders, which you can't, or else you'd do that rather than waste your time here. Forget your offline PCs, start looking for hidden cameras and microphones in your room and agents following you on the street.