This seems very doable with biblatex and Biber.
In fact the verbose-ibid style with a few standard options (autocite=footnote for footnote citations that get turned into endnotes with notetype=endonly) gives you almost what you need. The subsequent citations of verbose-ibid are "Author, Title", though, and not "Author, Year", so we need a few tiny tweaks.
\documentclass[british]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{babel}
\usepackage{csquotes}
\usepackage{endnotes}
\usepackage[citestyle=verbose-ibid, bibstyle=authoryear, backend=biber,
autocite=footnote, notetype=endonly, labeldateparts]{biblatex}
\addbibresource{biblatex-examples.bib}
\DeclareDelimFormat{nameyeardelim}{\addcomma\space}
\renewbibmacro*{cite:short}{%
\printnames{labelname}%
\setunit*{\printdelim{nameyeardelim}}%
\printtext[bibhyperlink]{%
\printlabeldateextra}}
\begin{document}
Lorem\autocite{sigfridsson} ipsum\autocite{nussbaum} dolor\autocite{nussbaum} sit\autocite{sigfridsson}
amet\autocite{knuth:ct:a,knuth:ct:b}.
Lorem\autocite{sigfridsson} ipsum\autocite{knuth:ct:a} dolor\autocite{knuth:ct:b} sit\autocite{knuth:ct:c}
amet\autocite{geer}.
Lorem\autocite{knuth:ct:c} ipsum\autocite{geer}.
\theendnotes
\end{document}

If you absolutely can't use biblatex, here is an attempt to implement a poor man's version of the above.
\documentclass{article}
\usepackage[authoryear]{natbib}
\usepackage{bibentry}
\usepackage{endnotes}
\usepackage{etoolbox}
\makeatletter
\newcommand*{\pblx@autocite@i}[1]{%
\@ifnextchar[%]
{\pblx@autocite@ii{#1}}
{\pblx@autocite@ii{#1}[]}}
\def\pblx@autocite@ii#1[#2]{%
\@ifnextchar[%]
{\pblx@autocite@iii{#1}#2}
{\pblx@autocite@iii{#1}{}[#2]}}
\def\pblx@autocite@iii#1#2[#3]#4{%
#1{%
\ifstrempty{#2}
{}
{#2\prenotedelim}%
\forcsvlist{\pblx@autocite@process}{#4}%
\ifstrempty{#3}
{}
{\postnotedelim #3}}%
}
\def\pblx@autocite@process#1{%
\ifcsundef{blx@citeseen@#1}
{{\frenchspacing\bibentry{#1}}%
\global\cslet{blx@citeseen@#1}\@empty}
{\cite{#1}}}
\newcommand*{\simplecite}{\pblx@autocite@i{}}
\newcommand*{\enotecite}{\pblx@autocite@i{\endnote}}
\makeatother
\newcommand*{\prenotedelim}{ }
\newcommand*{\postnotedelim}{, }
\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@book{appleby,
author = {Humphrey Appleby},
title = {On the Importance of the Civil Service},
year = {1980},
}
\end{filecontents}
\begin{document}
\nobibliography*
Lorem\enotecite[1][2]{appleby}
ipsum\enotecite[2]{appleby}
dolor\enotecite{appleby}
\bibliographystyle{plainnat}
\nobibliography{\jobname}
\theendnotes
\end{document}
biblatex. Would you be OK with switching from BibTeX tobiblatex? See https://tex.stackexchange.com/q/5091/35864. See also https://tex.stackexchange.com/q/25701/35864 for background and https://tex.stackexchange.com/q/154751/35864 for help with Biber. – moewe Aug 11 '18 at 16:23