I'd be surprised if she ate any flakes. Just drop the pellets down in front of her nose and stick around to make sure she gets 2-3 pellets at each feeding.
To get rid of her stress stripes...get rid of her stressors. This may mean your filter, and it may mean the other fish in the tank.
No one seems to be bothering her though... They do seem less bright than they were before so maybe its the move from the fs to here? And being at the fs in the tiny cups...
My filter is very calm so I dont think thats it either.
Some are bothered by ANY filter whatsoever...the slightest current will mark her up with stress stripes. Others, the current doesn't bother them hardly at all. Some are bothered by the very presence of other fish, and some love community tanks.
I have 3 female bettas in a community tank with other fish. I put their Betta food into a feeding ring and they eat it out of that. They also take food out of my fingers and have taught the female Ram to do the same! My girls get along really well with each other and the other fish. They sometimes show horizontal lines, if I have changed water, etc. but most of the time they are brilliant red, very gentle and lovely fish. And yes they do hang in the plants!
They aren't indigenous to an area that has water movement, so they aren't equipped to deal with it. Their body shape and the length of their fins also acts somewhat like a paper bag in a windstorm, or some similar analogy. Imagine trying to swim in a river against the current with a big baggy coat on. It's just not any fun...unless you're into that kind of thing. And Bettas aren't.
You might be best sticking to just the one you have, as you already have quite a few fish in the 10 gallon. I think some people keep about 6 female bettas in a 10 gall. but not with other fish.
I was told to have 4 or they would fight but my 3 are sisters and they arrived together and haven't fought up to now. They do have a lot of space and plants and hidey places so that might be why they get on so well.
Other people would probably be able to give you better advice, I don't know much about Bettas really - I just love them!
She IS lovely. And those are stress stripes, but they aren't very dark, and she hasn't lost very much of her color, so she's not traumatized, but she is scared.