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 by his father Indra. The same is confirmed in Uttar Kanda: Sarga 42:
Thereupon Indra begot on her a son named Vali for he originated from her hairs and Surya begot on her another son, who was named Sugriva for he originated from her neck. Thus the two highly powerful Vanaras being born, Indra went back to his region having conferred upon Vali an un ending garland of gold. And having engaged Hanuman, Vayu s son in the service of Sugriva, Surya too went back to the sky.
What happened to it after Vali was killed?
Vali gave it to Sugriva before his death:
The bounteous goddess of triumph ensconced in this golden pendant will completely leave it off on my death, avoiding the flaw of touching a corpse, isn't it... hence oh, Sugreeva, you wear it." Thus Vali spoke to Sugreeva. ~4-22-16