> It is absolutely possible to legally use React while going ahead and suing Facebook.
Only if it's possible to use React without infringing (or licensing) any Facebook patents. "Use" is not just about copyright.
(Which, as you point out, is plausible; there's just no reason to believe it's guaranteed to be true.)
OK, so I can legally use React-Native in my app while suing Facebook for unrelated patent infringement. I also can claim the patent they are suing me for infringing is invalid while continuing to use React-Native.
Would it be practically advisable to do so?
It is absolutely possible to legally use React while going ahead and suing Facebook. You do not lose your license to React by suing Facebook.
What you do lose is an explicit grant of additional patent rights - the grant given in the PATENTS file. Note that there is no specification therein regarding what patents might be covered; it's completely possible that none of Facebook's patents have anything to do with React. I haven't looked. in any case, patents are totally distinct from you license to use React.