> in what sounds like nonstandard manners
Not moving into the shoulder, that's illegal and dangerous.
Once I've overtaken traffic that is _actually_ slow, I move to the right lane. There is nothing nonstandard about letting traffic pass me on the left when multiple lanes are available. Because I am not tailgating, they have ample room to safely move in front of me (5 seconds at highway speeds is a very large distance).
> both of these increase the likelihood of accidents occurring
Speeding and tailgating is a process that occurs all the way to a destination - it is a consistent threat. Lane switching doesn't need to occur frequently and can be done well in advance when it is generously safe to do so - it is a momentary threat.
> by driving slower than other cars, you're impeding the ability for traffic to flow evenly
By tailgating or anything between safe following distance, you are not giving adequate room for people to merge into traffic. Traffic that is entering is forced to come to a near-stop because they can't find an opening, slowing down or stopping traffic behind them when they do eventually enter the highway.
Everyone traveling at an agreed speed, with ample room for traffic to enter and leave is traffic flowing evenly.
> slow driving
Driving at the speed limit is not slow.
> which doesn't really fit the adjective "courteous"
You have _moved_ that word completely out of the context that it was used in. "Courteous" is letting people in (which aids traffic flow, especially in gridlock).