Battery efficiency questions in the context of running

I've followed Gordon's advice, uploaded "gps only" on a fully charged banglejs2. I was able to race 2 hours with the run+ app and the wrist heart rate monitor running. Yeees ! A few questions still.

• Would it draw more power to use the btrhm instead of the internal heart rate monitor of the bjs2 ?
  
• I've asked in a different thread if it's possible to get more data out of a bthrm (ground contact time, stride length, vertical oscillation,...). Would it draw noticeably more power to get a bit more data out of the bthrm ?
  -I have "Wake on Tap" and "Wake on button" on on my watch ("face up" and "twist" are off). I noticed during my runs, that the screen lights up without me touching it. Is it related to "wake on tap" being on ? (actually going to answer myself to this question).
  Now I know the battery can last a half marathon, very happy with that. Next year, I will probably up the game to the marathon. Any chance the bjs2 can make it ? Are there some enhancements, tweaks that would help (considering there's a lot of time to think about it) ?