To what extent is anonymity through a Chaum mix compromised by using trusted mixes in the cascade?
I haven't seen a paper that addresses your (first) question specifically. One extreme example of a mix node that you trust is yourself, if you are participating in the network as well as using it. One paper that talks about the trade-offs of using yourself as a node in a mix network is: On the Economics of Anonymity. It sort of addresses your second question.

While Tor is not exactly a mix network, I believe it carefully chooses entry nodes based on individuals the network maintainers trust. In this case, all users have the same "trust network" and so it avoids the problem you are proposing to study. The drawback is you personally must trust the same nodes. 

As to your specific question, I haven't seen a paper that specifically addresses what you want but I also do not follow the area closely. Most papers on Mix Networks are catalogued here: http://freehaven.net/anonbib/ and so you could thumb through the titles for something that might be related.

Finally, I'm not sure you'll find the adversary has much power unless if they can observe large portions of the network, in which case there are more effective attacks (especially in Tor-like anonymity networks which don't actually mix or obfuscate packet timing).