Could someone explain to me why is PoSA v2 anonymous?
Ally has 2 PoSA addresses: Joe's and Node#1.
The transaction flow is the following:
tx#1 Ally is sending coins to Node#1 PoSA address together with Joe's PoSA address (how do you send the address in a transaction?).
tx#2 Node#1 is sending coins to Joe's PoSA address. (this was not (well) described in the diagram)
tx#3 Joe is sending coins from his PoSA address to Node#2 PoSA address
tx#4 Node#2 is sending coins to Joe's receiving address
 
Here is the problem I see: tx#4 is not anonymous as Joe's receiving address is linked with Ally's sending address through the above 4 transactions.
Just in case, here is the diagram with the possibly problematic line: http://i.imgur.com/UCMqsIr.jpg