Features in which the derived state involves a reduction or a loss of a character primitively present or prominent can occur repeatedly in otherwise unrelated lineages, simply as convergent adaptations to similar lifestyles. As I mentioned before, the holotype specimen includes only a single caudal vertebra, from which the long tail and its haemal arches have somehow been inferred. Not very likely, I'm afraid. Not very much. Feathers are entirely plausible for the creature.
Proofs occur only in mathematics.
No collection of anatomical features can ever conclusively prove or disprove a phyletic hypothesis. Paleontological data at best can only affect the likelihood of an evolutionary event. But having looked over some of the recent papers on dinosaur-bird relationships, I've changed my mind. Dromaeosaurids were by no means giant, flightless archaeopterygids, just giant, flightless dino-birds of a more primitive grade.