How was Heathcliff able to use the 'assist mod' mid-battle?
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.