Chapter title spacing and underfull vboxes

I am trying to create a document that has the following properties:

• line spacing is non-standard but constant

• paragraphs have no perceivable vertical margins between each other

• text height is automatically adjusted to exactly fit the contents, based on a page of only paragraphs

• chapter, section and subsection etc. whitespace is expanded to fill pages

However, this does not work as intended. The spacing affects the relevant commands as shown by the fact that spacing is changed, but the glue seems to have no effect, as the paragraphs do not reach the bottom, and there are errors about Underfull \vbox (badness 10000) on every page with a chapter title.

\documentclass[twocolumn]{memoir}

\usepackage[latin]{babel}
\usepackage{lipsum}

\renewcommand{\baselinestretch}{1.15}

\isopage[12]
\setlrmargins{*}{*}{1}
\checkandfixthelayout[classic]

\setlength{\parskip}{0pt plus 0pt minus 0pt}
\setlength{\beforechapskip}{0pt plus 1fill minus -.2ex}
\setlength{\midchapskip}{0pt plus 1fill minus -.2ex}

\begin{document}

\chapter{Lorem ipsum}
\lipsum[1-20]
\chapter{Dolor sit amet}
\lipsum[11-30]
\lipsum[21-40]
\chapter{Sed ut m\ae cenas turpis}
\lipsum[31-50]
\chapter{Congue marinam}
\lipsum[41-60]
\chapter{Perspiciatis nullam nibh}
\lipsum[51-70]
\chapter{Duis convallis orci id}
\lipsum[61-80]
\chapter{Nam viverra ultricies}
\lipsum[71-90]

\end{document}

• 0.0000001pt and 0pt are the same – egreg Jun 5 '17 at 12:02
• What do you mean "the paragraphs do not reach the bottom"? \beforechapskip affects the spacing above the chapter's title, on the page that begins a chapter. It will only affect pagination on that page, if it does. You should not expect that, because of this, the last page of the chapter will extend to the bottom margin. – nickie Jun 5 '17 at 13:14
• The pages would be about half a line longer without redefining the lengths. With the redifinitions, the pages containing only paragraphs are the same length. About the last page of chapter, that's okay, as that could only be changed by altering the contents of the text itself anyway. – svenper Jun 5 '17 at 14:08