Kostornaia's injury was to her wrist, not exactly something that's going to impact her ability to skate, while against Usacheva, Sinitsina does have a stronger case because she's not recovering from a serious injury; per my comment about Kostornaia's coaches politicking power, it applies to Usacheva as well and not just politicking power, but a track record of successful athletes winning at the highest level (including bringing them back from serious injury) whereas Sinitsina's coach doesn't have near the history that Usacheva's does.
And yes, Samodelkina (who is on the junior team this year), Petrosyan, and Muravyeva have no senior international experience - Sinitsina's experience is minimal and given their ages and technical arsenal they have better future prospects than her.
You disagree I understand, but IMO I understand and agree with the selections for Kostornaia, Petrosyan, and Muravyeva over Sinitsina. Usacheva, while I do like her as a skater, I think was pushed over Sinitsina based on politicking power of her coaches.