Reflection of sound waves from the open end of an organ pipe & relationship b/w nodes & pressure

Your first question is answered by this Open University article. To summarise, consider a low pressure region travelling along the tube towards the open end. The air outside is at atmospheric pressure, so when the low pressure region hits the end of the tube air from the atmosphere rushes in and creates a compression wave heading back down the tube. The opposite happens when a high pressure region hits the end of the tube.

The reflection of a sound wave at the end of a tube is an example of an impedance mismatch.

This also bears upon your second question. At an open end the wave inverts i.e. a reflected pressure peak becomes a trough, and a trough becomes a peak. This is in contrast to the closed end where a pressure peak reflects as a peak. This means the pressure changes are lowest at the open end and highest at the closed end.

The reflection at the open end is not 100%: some energy escapes. Actually this is obvious, because if no energy could escape an organ pipe wouldn't make any sound. The sound we hear is the energy that has escaped from the open end.