Hi Onno,
Good explanation!
For the first part, I know -36 dBm is a very great signal for normal situations, but in this case the laptop and AP are kissing each other. Therefore as you said the RSSI detector at the receiver probably can't handle such a signal and may just show you the maximum it can do or handle.
For the second part you say the graph shows SNR, since dB is the correct unit. But the Instant GUI shows the AP transmit power in dB, when it should be dBm, look at this screenshot:
The Aruba Instant User Guide says for the Client Tab:
Signal—Current signal strength of the client, as detected by the IAP.
But it is not clear. Do you know a command that shows the signal strength received by the AP (not SNR)? I have been searching but I didn't found anyone.
Anyway searching old posts I have found one where Colin says "The numbers are the signal strength that the AP sees from the client. If the number is positive, it is SNR. If it is negative, it is dbm."
http://community.arubanetworks.com/t5/Wireless-Access/Viewing-client-signal-in-Instant/td-p/297000