in answer to your first question the body consumes old cells via autolysis. The body is very selective as to what it uses for fuel in the absence of external nourishment by using the least essential first, and preserving the most vital until last.
These cells are replaced when the fast is broken with the nourishment taken at that time and thereafter.
Celery juice with wheat-grass juice would be fine in breaking a fast, even a long fast, but the bodys receptiveness to nourishment depends on whether the return of hunger has been reached or not. In the former the body will easily digest and assimilate food, but in the latter, more care is the order of the day.
Green-juice in-between fasts is commendable in my opinion.