from seedling you need more grow time, usually atleast 6-8 weeks before you can think about flowering.

i believe you are talking about revegging the plant after harvest, and yes you can harvest all but the bottom of the plant and should beable to nurture it back to a healthy plant. depending on strain, age, stresses, ect depends on how long till it has revegged and come back to good health.

personally i would recomend finding a good mother plant and take clones from her and flower those clones.
archive

theres all the info you need about clones and mother plants