When other "g" clients are in use on the channel, some clever things are going on in the background. As far as your own router goes, if "g" is in use by anyone, the transmitter obviously can't use two methods at once, so it will back down to 54 Mbps or lower "g" or even "b" speeds to service that client. That means it will be continually changing it's channel width, transmit method, and speed. That will account for the readings you see jumping around. When none of your own clients are active you will often see the speed changing - the router seems to change parameters depending on what else it can hear on the channel.
Most AP's seem to be quite a lot weaker on this latest graph, which is good news.
"DANGER" is MUCH stronger than the other routers there, so it has a good chance of working at full speeds - the interference mitigation setting allows for the coexistence of the other stuff on channels 1-7. Interestingly, at your location, MING-PC could affect half the band, NETGEAR 2WIRE390/373/614 on channels 8 and 10 are capable of affecting the other half. This is a very common scenario because of the ability to set odd intermediate channels, which after all isn't documented in the router setup manuals!
And don't forget that for every ROUTER you see, there may be one to 30 or 40 CLIENTS on the band which you can't see with InSSIDer. Therre may be several hundred active transmissions. It's remarkable anything works if you think about it. But then along came "N" and made it even worse.
This mess should never have happened - I guess some business politics got in the way of sound engineering practice when the specifications were revised to allow 40MHz channels on 2.4GHz. Some manufacturers originally recognized this is going to cause bad publicity for their products and didn't allow it.
Anyway, you've got the idea now - we just have to do the best we can.
Oh, BTW - please post the model number of your TP-Link USB adapter, which is giving nice readout with InSSIDer above the old limit of -50dB.