Is it permissible to perform the Farewell Circumambulation (Tawaf al-Wada') during Hajj upon entering the boundaries of the Masaa (the area for Sa'i) due to overcrowding, knowing that the circumambulation will be performed from the rooftop?

The Masaa (area for Sa'i) is not part of the Mosque. Among the conditions for the validity of Tawaf is that it occurs within the Grand Mosque (Masjid al-Haram). Therefore, whoever performs Tawaf outside the Mosque, even for part of a single circuit, their Tawaf is invalid, and they must offer a دم (dam, expiatory sacrifice). The minimum for this is a sheep to be slaughtered within the Haram area and distributed among its poor. It is permissible to appoint someone to carry this out on one's behalf.