How was Heathcliff able to use the 'assist mod' mid-battle? Heathcliff explained to Kirito that he had to engage the system’s assist mod when defending (against Kirito's attack) because he's (Kirito) so fast. But how was he able to automatically access it? I know that he is the GM but we’ve seen that even GMs have to either utter a voice command or navigate through the menu in order to instruct the system.
I think the reason why Heathcliff was able to do so lies in the fact that NerveGear works by scanning the brain wave. It receives input by reading the brain wave as the person equipping it thinks about something. For example, when we talk, the words are formed in our mind and the brain sends pulses to our mouth and voice cord to produce sound matching the words formed. This pulses is what the nerve gear receives as input, and at the same time, prevents the actual organ from receiving it.

It is possible that for his account, he has a feature where he can activate or deactivate game feature simply using his mind. That is, he can command the system by thinking about it. No need for him to think of moving his hands and access the control panel.