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 ...
It should not be possible as, in Ramayan, there are various references to Himalaya like Dronagiri, where Sanjeevni was found, and rivers like Ganga which originates from Himalaya.
The origination of Himalaya after the Indian subcontinent collided with the Asian continent must have taken millions of years. The age of these people were long, but not that long....