Sugriva narrates his enmity with Vali here in Ramayana, which was already narrated in detail by other member.
The reason for Vali not getting appeased may not be anger alone. If it was so, it will wane away with the passage of time.
However, Vali, apart from banishing Sugreeva from his kingdom, captured Ruma, the wife of Sugreeva, and enjoyed conjugal ...
Can someone help explain who is Mahendra?
It's another name of Indra, this is clear from this alternate translation:
Having addressed thus in a rage, Vali removed the golden garland gifted by Indra, his father, and stood ready for the combat. ~Source
Any references to how he obtained this pendant?
In above shlokas also, it's mentioned that it was given ...