Well, trusting more than one issuer for the same currency turns yourself into a gateway.
Hmm...no. A gateway is an entity that self-issues currencies in the Ripple system.
if it only takes one untrustful person, to compromise the complete trustline/network?
No. Schwartz explained this elsewhere really well. Even if there is someone somewhere in the chain that you don't trust, all Ripple payments are atomic. They either go through completely, or fail. If it goes through, you get exactly the balance you asked for (or better). If it fails, no balances are changed.