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....
Yes, it is true that Vali himself ordered Sugriva to stay vigilant at the entrance of the hole, through which Maayaavi entered.
इह तिष्ठ अद्य सुग्रीव बिल द्वारि समाहितः |
यावत् अत्र प्रविश्य अहम् निहन्मि समरे रिपुम् || ४-९-१३
Then Vali said to me, "Now you stay here on the alert, Sugreeva, at the entrance of this hole till I return on destroying ...