Can we avoid third party trust for mobile / desktop wallets with respect to lightning network channels?
(disclaimer: I'm not an expert on Lightning)

I believe the design for Watchtowers (the services which watch for other parties trying to push an old state to the chain) is such that they're not actually given the channel close transaction.

Instead, they're giving half of the txid to watch for, and an encrypted version of the channel close, using the full txid of the watched transaction as key.

This means they are not actually able to dishonestly close your channel. They can only do so when an invalid state update is published.