The Fallen need to partner up with a mortal being with free will, but not all wills are equal. Some are broken, weak and vacillating easily dominated by their Fallen. Others not so. We have seen Marcone’s will, he cannot be bullied, cajoled or seduced by a Fallen they would have to partner with him on less than equal terms, he can threaten to lay down his coin and he would do it, and place that coin somewhere humans couldn’t get at it for a very, very long time.
Lash started to get to Harry, by subterfuge, when Harry was aware of her he was able to get the upon hand and he changed Lash.
Marcone can dangle in front of Namshiel a couple of things, freedom from Nicky for one and the indulgence of Namshiel’s quest for magical knowledge for another. Both go with Marcone’s interests.