here is one example of how the Query optimizer can use FK metadata.

Now on to your non-trusted issue.

The reason replication marks them as non-trusted is because you could potentially use multiple subscription streams which do not guarantee order.

Secondly FK per se is not that big a deal for a subscriber that is only going to read, as long as you have indexes covering your joins you should be fine and that is not the "end-of-the-world" 🙂