Lowest power way to send button presses to a PC in realtime

There are a few power figures here: https://www.espruino.com/MDBT42Q#power-consumption

But basically it shouldn't make too much difference. Behind the scenes, HID is just notifying on a characteristic anyway, just like you would be.

What will be hurting you is the connection/advertising interval though. That's how fast packets are sent. Best bet is:

// For when you're connected
NRF.setConnectionInterval(200); // in milliseconds - you can go even higher than this
// For when disconnected
NRF.setAdvertising({}, {interval:500}); // default is ~300. Higher values use less power, but make connecting slower/more difficult

Hope that helps! When connected with a low connection interval you should be able to get down to 0.04mA or so