I advise my patients to use their crutches until they can reliably bear weight without the knee giving way. Most are stable by two weeks but occasionally a patient will need to use them a bit longer.

Yes - the instability you are feeling is likely related to the fact that your muscle (quadriceps) group in your thigh is shut down post-surgically. This is extremely common. Just give it time, use crutches if you need to so that you don't fall, and work hard in physical therapy to get your strength back!

You need to discuss this with your treating Doctor. He/She would know Best.

Yes. It feels like it's going to give out because your quad muscle is weak. The quad is weak because of the injury and the surgery. Do straight leg raises 100 times a day for 10 seconds each. This will strengthen your quads.

Hope it helps!!

Dr. Bose

Yes, it is normal to feel like it is going to give out 2 weeks after surgery due to weakness. That feeling will go away as your leg gets stronger. 

Hard to say, but keep icing it and elevating it. If weight-bearing is painful, please consult your surgeon.
Good luck!

Two weeks is too early to make an assessment about your knee. It takes approximately 6 to 12 months to fully recover from major knee surgery.