KomaScript Book: How to have Chapter Headings formatted just like text?

KomaScript Book: How to have Chapter Headings formatted just like text?

Might seem like an odd thing to do, but so far it's proven beyond my
novice-level Latex abilities, and related posts I've looked at on this
site seem a little more complicated than what I need. In particular, I'm
writing a dissertation, and my university requires the following:
12pt Times New Roman
Double Spaced throughout
Chapter Headings are distinguished only by centering, and the use of all
capital letters (this includes the headings for things like the table of
contents), and the use of what in komascript amounts to the
chapterprefix=on option
The komascript manual was pretty clear on how to get headings centered.
Still, my headings are formatted by default with the following undesired
attributes:
Some font other than Times New Roman
Bold-Face
Huge white space above the heading (I need no space but the top margin)
So, how to I tell komascript to back off a little, and not add all the
extra formatting to the headings?
Here's my working template so far:
\documentclass[12pt,letterpaper,oneside,chapterprefix=on]{scrbook}
%Margins
\usepackage[letterpaper,left=1.5in,top=1in,right=1in,bottom=1in]{geometry}
%Type
\usepackage{fontspec}
\setmainfont{Times New Roman}
\usepackage{unicode-math}
\setmathfont{xits-math.otf}
\usepackage[doublespacing]{setspace}
\setlength\parindent{0.5in}
%Other
\usepackage{microtype}
\usepackage{lipsum}
\setkomafont{chapter}{\centering}
\begin{document}
\frontmatter
%Flyleaf
\clearpage\thispagestyle{empty}\mbox{}\clearpage\setcounter{page}{1}
\chapter*{Abstract Title Page}
\chapter*{Abstract}
\chapter*{Copright Page}
\chapter*{Title Page}
\tableofcontents
\listoffigures
\mainmatter
\chapter{Introduction}
\lipsum
\appendix
\backmatter
\end{document}