She'll have to get through the SP first, something she hasn't done all season long IIRC.
People are saying Polina's a great bet for that 3rd spot but when you look at it from an international standpoint, she doesn't have any more rep with the judges than Courtney, Samantha or Mirai. So far this season, Polina has been less than mediocre TBH. Her highest SP score is 50.32, and her highest FS score was 112.83. Both of her totals at her GP this season were 161+; Samantha and Courtney's top scores were 174+, Mirai's was 165+. Internationally, Polina is not getting any special attention from the judges...I'd say that's in large part b/c of how badly she's performed in the SP but also b/c she's frequently hit with URs and dings/edge calls on her 3Fs. I do think she'll get much more favorable calls at nationals b/c she's the silver medalist and the USFSA really wants her to do well.
It's obvious the US needs both Ashley and Gracie to go to worlds. Together the two of them are very capable of maintaining those 3 spots. Essentially, whoever goes in as the 3rd American really won't be expected to do much. For that reason I think the spot should go to whoever has the best competition. I could really do without the propping this year but, of course, I know that's impossible to avoid at nationals. I just don't want to see another Agnes Zawadzki situation (low international scores, inexplicably high national scores). If Polina skates well, she deserves to score better...but if the judges ignore her UR and edge issues and score her through the roof, that's not going to help anything.