Are pneumothorax and atelectasis the same thing?

No. Pneumothorax causes atelectasis but ther are other causes of lung collapse.
No. Atelectasis implies that some aerated areas pf the lung have deflated like an empty balloon. This shows up as an increased lung tissue density on x-ray. Re-inflation is often as simple as shaking loose some mucous plugs. A pneumothorax occurs when air gets between the inner wall of the ribcage and the lung, pressing the lung tissue inward.