Is routing decentralized and trustless:
Generally, yes. It uses sphinx onion-routing to prevent any peer along the way to know origin/destination of payment.
Also note that Information of channel balances can leak if routing attempts are used to probe liquidity along a route.
Routing is not robust against sybil attacks, as compromised nodes can determine routes by reconstructing path via payment_hash in the htlc output scripts.
It is, however, incumbent upon the channel operator to ensure the lightning implementation is always synced with a full-node. Only this way can it detect a cheating channel counter-party and broadcast a penalty transaction. Watchtowers may provide vigilance as a service but they are not trustless
Also, note that the lightning network is a separate p2p network.
Discovering topology occurs with peers gossiping.