<<What sort of fossil would cinch either side? If there definitively *was* a
biplane stage, what might we one day find?>>
<At least two double-decked basal bird clades.>
Not neccessarily true. [...]
<<If there definitively *was not* a biplane stage, what would we find instead?>>
<At least two single-decked basal dromaeosaur clades ("dromaeosaur" here actually meaning "everything closer to *Microraptor* than to birds").>
We would have to find out that *Microraptor* was not, in fact, a biplane.