2nd Question: How can the companions of the Prophet, who had fought with the Prophet, turn back on the allegiance that they made at Ghadeer?  The simple answer is that this wasn't the first time that they had turned back on their allegiance to the Prophet. 

Here are three examples: 

1- Breaking the Beyah al-Ridhwan (Allegiance before the Treaty of Hudebiya):   When the Prophet went to Makkah for Umrah with 3-5K companions, Quresh didn't allow him to perform Umrah. Prophet asked all Muslims to renew their Beyah/allegiance to him under a tree outside Makkah. All of them agreed to accept Prophet's decision whether to fight or agree to a truce. This resulted in the Treaty of Hudebiyah. However, many turned against the Prophet in 2 days including Umer.   The Prophet ordered the companions to break their Ahram and offer sacrifice. They refused to do so without going to Makkah. The Prophet was very upset and went to his tent where Hazrat Umm-e-Salma was accompanying him. She asked about what is making the Prophet sad. Upon learning, she recommended that the Prophet should break his Ahram and offer sacrifice. Once he, Imam Ali and a few companions did that, other followed.  

2- Breaking the covenant at the Battle of Khyber  On Day 1, Abu Bakr was given the flag and went to fight with the army but retreated. On Day 2, Umer was sent but retreated. This continued for 39 days. Then on the 40th day, the Prophet gave the flag to Imam Ali AS, who didn't retreat. Rather defeated the army of Khyber.  In Surah Ahzab, Allah mentions that the Muslims (companions of the Prophet) have their allegiance that they will not return without a fight but kept running back for 39 days. Thus breaking their promise to the Prophet.  

3- Rebellion against Usama  The top companions declined to go fight under the generalship of Usama bin Zayd when the Prophet asked all the senior companions to leave Madinah with the army of Usama except for Imam Ali AS. They argued that Usama was young.   The Prophet condemned them and said that you did the same against his father Zaid bin Hairitha when he was made the leader in the battle of Mutah  The people did the above things and broke the covenant of the Prophet because of their nature. 

There are two reasons why they ignored the allegiance of Ghadeer:  1- The thought of the Prophet as a regular human like themselves who only became a Prophet when he received the revelation. But Allah says in the Quran that Prophet is a Prophet all the time  2- It was part of their nature to be attracted towards power, authority, and Pride. They wanted to get absolute power, pride, and authority by removing Imam Ali AS from Khilafat.   They showed their inclination towards the above even before Islam. For example, when Quresh rebuilt Kaaba and the question was raised about installing Hijr Aswad, they all disagreed and were about to fight/kill each other. But then the Prophet resolved the issue by putting Hijr Aswad on a piece of cloth that all leaders held so that it was brought close to the Kaabah and then he himself installed Hijr Aswad.