We've had a few discussions about each of the topics/issues you've raised. Not sure there is a one-size-fits-all answer to each question.
I carry a recorder (SONY PCM-M10) with me in a backpack belt pouch. It is not on when I am en route to a location. One demerit. Once there, I turn it on and either have it in my hand or in my chest pack with zipper open. The SONY has a 15-second buffer and I can capture something if I hit record within 15 seconds of hearing something which provides a little forgiveness for my somewhat sloppy standards.
We've talked about where to carry a recorder or microphone while walking. I've never taken the time to rig up an external mic to the backpack itself which I know I should do. Second demerit. Ideally, I'd run a wire and have the mic somewhere at the top of my backpack but I've taken the initiative to do so. The simple truth is most of the things I've heard have happened once I've settled in. No excuse though as I would likely miss a whoop if I heard it while on my way to a location. Third demerit.
In the evening, before retiring to the tent, the recorder is placed some distance from the camp and left to record throughout the night.
There are several folks here who really know the ins-and-outs of audio recording and have done a good job at maximizing their chance of getting a good recording. Hopefully, they'll weigh in.