Will an iOS device fall back to 802.11a when 802.11n is unavailable? I was told that iOS devices defaults to 802.11a which runs on 5 GHz band when 5 GHz 802.11n is not available. Have anyone experimented with that?

How does negotiation between access points and client devices work?
The wireless chipset samples all spectrums and chooses the frequency that has the highest potential speed with a joinable SSID and won't generally switch to a different network unless there are errors or a drop. 

Since the protocols are different there isn't so much a negotiation as a decision tree which of the several channels to join.