Is it permissible for the pilgrim performing Tamattu' to stone the Jamarat before zawal on the twelfth day of Dhu al-Hijjah, and what are the consequences if one is unable to perform the Farewell Tawaf due to overcrowding and a scheduled flight?

Throwing the pebbles before zawal on the days of Tashreeq and the first day of Nafr is not valid according to the majority of scholars. You are required to offer a damm (sacrifice) for abandoning it, which should be slaughtered in Mecca and distributed among the poor of the Haram (definition). You are also required to offer a mudd of food for abandoning the stay overnight on the thirteenth night, and another damm for abandoning the Farewell Tawaf (definition).