Can a trade offer be the last leg of a Ripple transaction path? If so, do transit fees apply?
You have a slight misunderstanding about payment paths that is confusing your reasoning. When a payment path ripples through an account, that account receives an asset from the previous link in the path and issues an asset to the next link in the path, not the sender.

So the answer to your first question is that no trust is needed since Alice never holds the currency. An asset, other than XRP, can only be delivered to a person who created a trust line or an offer that agreed to accept that asset. But she never holds it.

And the answer to your second question is that there is no second transit. Alice never holds the asset because she never agreed to. The asset is delivered to the next link in the payment path.

To use a more realistic payment path, consider a case where Alice holds USD issued by Snapswap and wants to pay Bob a Bitcoin. Bob must have a trust line or this is not possible, so let's say he has a trust line to Bitstamp with a limit above his balance. The full payment path could be:

Alice/USD -> Snapswap/USD -> Order book -> Bitstamp/BTC -> Bob/BTC

Notice that only an order book can change the currency. Alice returns USD to Snapswap, and in exchange Snapswap issues USD to whoever placed offers in the order book. The offer owner in turn sends BTC back to Bitstamp who in turn issues them to Bob. Nobody ever holds an asset they didn't agree to hold.

This makes it easier to understand how transfer fees apply. If Bitstamp charges .2%, then the market maker must send 1.002 BTC back to Bitstamp in order to deliver 1 BTC to Bob.