As per Mahabharata. Manthara happened to be an incarnation of Gandharva by the name of Dundubhi, who followed directions from Lord Brahma
From Mahabharata's Vana Parva
And at this, the gods, the Gandharvas and the Danavas quickly assembled to take counsel as to how they should be born on earth according to their respective parts. And in their presence ...
Rama Rajya speciality is described at the end of yuddha kanda.
न पर्यदेवन्विधवा न च व्यालकृतं भयम् |
न व्याधिजं भयन् वापि रामे राज्यं प्रशासति || ६-१२८-९९
raame = while Rama; prashaasati = was ruling; raajyam = the kingdom; na vidhavaaH paryadevan = there were no widows to ...