The proximally mobile, distally stiffened tails of dromaeosaurids and birds are pretty rare among dinosaurs. Ostrom argued that the dromaeosaurid tail was a balancing pole during combat, allowing the raptor to balance on one foot while slashing with the other. Well developed on modern birds, these are absent in Archaeopterygians. They may have been cartilaginous in Archaeopteryx. Stiffening the rib cage, as you say, would be useful for grasping predators as well as for fliers.
Evolution works by exapting previously existing structures for new uses.
I would suggest that the arms of these theropods were NOT folded to increase speed, but were instead folded for use in a praying mantis-like grasping system. A big breastplate only tells you that the forelimbs were powerfully muscled. Furculae are not yet officially reported in dromaeosaurids, but their presence elsewhere in Tetanurae strongly suggests that they were there. There is NO reversal of arm length in Dromaeosauridae. That only proves that they were related, not how.