What if she refuses though after I pay to her? Is there a way to make it atomic and trustless?
What if she refuses after you pay to her on the current protocol (forget about LN)?
The problem is that someone might not want to route your payment because it has an impact on every channel that takes part in routing. You can set the fees really high if you don't want to bother with re-balancing your channel in the future. This should not be a problem if you have only one channel open. I have read it once again, I described a different problem.
What if she refuses though after I pay to her? Is there a way to make it atomic and trustless? If so, can anyone point to the description of low-level protocol implementation of this?
You would have to find another route and you would get your money back. We can't force people to route your payment, we can observe the same thing by looking at Bitcoin mining. Mining pools can choose which transactions they want to include in the block (e.g. the ones with higher fees)