Can a fetus develop a cleft lip after 21 weeks? Just had my 35 week ultrasound and there's a big line in the upper lip in her face pictures. Worried

Not at all. The lip & palate structures form and finish between 5 & 12 wks. There is no way they can undo what is complete. A well trained sonographer can detect a cleft by 16-20 wks. Any line you see likely represents a fold of skin caused by the various pressures of the womb on the baby. Your doc should be able to get these pics & explain them.
No. The direct answer to your question is no. Cleft lip is a defect of development that occurs early. It is typically discovered at your 20 week ultrasound, and if good pictures were obtained at that time, then you have little to worry about. It is not something that can occur at 35 weeks, but would conceivably be discovered at that time if not seen earlier.
Cleft lip. It is possible. Although this occurs in earlier stages. For now, you should just wait and see. Hopefully your child won't have one. But even so, there are many good surgeons and centers that can correct the problem.