You've now changed the situation quite significantly. In every case so far, I've been clear in arguing about multi-lane roads where the road isn't fully congested. This isn't some made-up scenario cherry-picked to prove my point: this is exactly the case I'm concerned about, and the one I understand you to have implied in your initial comment. You're now concerned with a one-lane highway.
In nearly all cases I've argued that the slower vehicle should merge right, not speed up. The only case I've suggested increasing speed is when the driver of the slower vehicle doesn't want to modulate speed moving between two lanes to allow faster-moving traffic to pass. You're now concerned with the slower driver needing to speed up.
Given you're no longer contesting the original supposition, should I assume your agreement? You've now moved on to specifically construct a situation — significantly different from the original — where "it's possible the faster driver ought to slow down rather than the slower driver speed up." You're asking me to argue and potentially agree to this new situation, while not once having conceded or even shown understanding (or asked for clarification) of any of the points I've made so far. That said, I'll continue to do my best to argue in good faith and respond to your new situation.
On a single-lane road, a driver should drive the speed they're comfortable with the conditions, including traffic. A following vehicle will need to slow down to avoid rear-ending the lead vehicle. Absolute speeds and number of vehicles above 2 are irrelevant.
If the following vehicle is tailgating, the lead vehicle can maintain their speed, speed up, or move out of the way depending on opportunity. Those are the things they can control. They're not under any obligation to chose one over the others; it depends on their assessment of the situation.
Practicalities aside, just thinking of the ethics.
The rules of behavior really don't change. If everyone were behaving legally and safely (within the min and max speed limits, safe following distances), you still should move right to allow faster vehicles to pass on the left, and for the same reasons: prevent passing on the right, prevent congestion, both of which decrease risk of collision.
At this point, I can't think of anything else to add, nor am I interested in discussing new tangents. I sincerely hope you consider changing your driving behavior, as it will improve traffic flow and be safer all around. Be well, and drive safely.