I wouldn’t recommend using the backpack strategy unless she has a way to do a safe grip (it could be stolen by said person). My best advice is to never give them attention. Do NOT make eye contact, do NOT acknowledge their existence, do not talk back. I’ve traveled 28+ countries and many cities alone as a woman. The best advice I can give is to just give them a cold shoulder. They will move on if you do. It is a possibility that they get more “triggered” and bother you more but in my many experiences that is very rare.

And also crazy doesn’t mess with crazier. If you speak another language use it to say random things or legit speak back in gibberish. In worst case scenario that they legit don’t leave you alone and it is getting unsafe, cause a major scene. This will bring attention to the situation to other bystanders and they will usually help and/or the creep will leave.

Like other said headphones could be a good idea, just very low volume or no audio at all to be able to stay alert. Aisle seat if possible or sitting next to other women. One more advice is that you can order pepper spray to an Amazon locker in Arlington and pick it up there. It is very hard to find it and buy it in DC.