Interesting thread!
I think there are differences across the events as well as the scenarios the OP lays out.
At PR-level fitness, I could go to the track/road alone and time trial a time the same or very close to what I could do in a race. At least in distances 5k-half marathon, where on a good day with the pace dialed in I am flat-out anyway.(Being a sub-sub-sub-sub elite hobbyjogger (16:xx/34:xx/1:16), I am often 'racing' in no-man's land, anyway so the mindset of hammering alone is the norm.)
A marathon is something else. I can't imagine running one outside of an event at all, let alone a PR effort. My experience with the distance has been that other runners and even small crowds in the back 10km make a huge difference. Watching the empty roads of the Dubai marathon all I could think is that I would never want to run it, one big lap up and down a freeway alone.
On the track, racing to the line is #1. I actually think a TT might be faster for me, though, than an out-the-back or strung out solo situation in a race, which is absolutely miserable in a track 800/1500. (I can't speak to a solo win on the track since I have never been far ahead in my rare wins!)