Is circumambulating (Tawaf) from the upper floor while entering part of the Mas'a area to avoid crowding valid, and if not, what are the consequences?

The Masaa (area for Sa'i) is not part of the Mosque. Among the conditions for the validity of Tawaf is that it must be performed inside the Mosque. Performing Tawaf above the Masaa without a valid excuse invalidates it. However, severe overcrowding is considered an excuse, so your Tawaf is valid, Allah willing.