RE: Throughput riddle
Responding to one part of your mail. The other parts have been covered in other responses:
> I was originally using one of the common 1/6/11 channels, and I switched to 3 since I saw a lot of other stations on those channels.
> This may have resulted in some improvement, but I'm still stuck locally as above.
Be aware that by doing this you have created interference with even more wifi networks then before. See https://en.wikipedia.org/wiki/List_of_WLAN_channels
If you select just channel 1, 6 or 11 you will have only interference with other networks on that one channel. By selecting an "in between" channel 3 you now have interference from both the channel 1 and the channel 6 networks.
Usually best is to see where the weakest networks are, on 1, 6 or 11 and place your wifi there.
In the 5GHz band there a lot more non overlapping channels and because of the lower reach the chance of interference is lower as well. However, because of that you might need more transmitters as well.