Completely open to collusion. All you need is a significantly large number of nodes accepting requests to be guarantors, and since there's no barrier to entry this is trivial.
The barriers are minimum coin holding requirements. So a cheater can't have many nodes there. Also, the whole idea is that most nodes are honest. If most nodes are bad, then you can't do anything whatsoever, and in which case the trusted centralized system is the only solution.
Moreover, guarantor doesn't do much, guarantor only gets involved if there's a disagreement between sender and mixer. If there's no disagreement (as should be in most cases), guarantor does not even participate the decision on distribution.
- "Among all the online coin clients, if some minimum requirements are met (e.g. with minimum amount of coins in the balance, and with minimum 2 addresses in the wallet, etc), the node will advertise itself as a service node." Creating two addresses is trivial, and even if you use address signing to verify a balance it's trivial to move funds out and into another address to advertise that node, and then bounce it back again. Sybil attacks aren't prevented by these minimum requirements, and higher the requirements just created a high barrier to entry for a "service node", and thus a reason for those running service nodes to knock others off the network.
Creating addresses are trival, but if the minuimum coin requirements is 10K coins, then a cheater can not make many fakes nodes. Transfer funds back and forth is not an option, as the validity of the service nodes can be checked periodically (and most likely when the receiving node process the message). Also it is likely just before the service request the fund balance can be checked.
What you said are non-issues and can be fully prevented by the algorithms. And supercoin did not state the concrete requirements for it, we can see when they publish them.