I'm having some trouble understanding some of the grammatical constructions found in the following sentence which was taken from an article entitled "горшки на коммунальной кухне". Here is the link: https://lenta.ru/articles/2016/12/30/usrfmilitary
Российским властям не впервой пользоваться методом «обнуления» накопленного в двусторонних отношениях негатива, когда, грубо говоря, все помои оптом сливаются за шиворот уходящей вашингтонской администрации, а отношения с новыми людьми начинаются с чистого листа.
First of all, why is Российским властям in the dative plural?
Secondly, I am not sure what role негатива plays in the first part of the sentence. Is it associated with накопленного? My attempt at translation is as follows: "the Russian government is not the first to use the method of equalization (not sure about translation of обнуления. I assume it to be something along the lines of maintaining nuclear parity from the context of the article) having been accumulated negatively in two-sided relations, when ,roughly speaking,....
Finally, I am not sure how to fit за шиворот into my translation of все помои оптом сливаются за шиворот уходящей вашингтонской администрации. My basic understanding is that all the "garbage" created by the Obama administration with Russia is "going out the door" once Trump comes into office. That is to say, there will be a clean slate once Trump and his people take over.