Re: 802.11b Mix Mode?
 

No, looks like it's not enabled.

Might be an idea to get another 802.11b only device (not sure if that's feasible) and see if that connects to your router. Def sounds like an incompatibility somewhere, but wether the router or phone is the prob I'm not sure.

Re: 802.11b Mix Mode?
 

I have a different setup, but on mine the mode on the client is set in: network connections; properties; configure; advanced.

In there I get a load of options for the wireless set up including IBSS 54g mode. Set to auto it works b or g. On the router its also set to 54g auto.