MESI Protocol Invalid cache line is attempted to be stored?
INVALID means "not in the cache," so there is nothing to flush. When you mark a cache block as INVALID it means that the data and the tag are not relevant.

Similarly, for your second question.  The line is not in P2's cache, so it should treat the line the same way it treats any other cache miss: (if it's a snoopy coherence mechanism then it should broadcast a message to all the other caches.  If it's a directory based coherence mechanism then it should send a request to the directory.)