rubber lengths in coffins and aligning to the bottom baseline of a paragraph

Question

I want to obtain a certain layout, illustrated below: at some point in a line, I'd like to "switch to paragraph mode" (just like a parbox does) and then at some other point I want to resume back to normal full-width typesetting. In my case, the idea is to get a ToC-like look using dotfill, but the question stands for regular text too, I suppose.

Using the xcoffins package I got this partial solution (MNWE below) which is neither very elegant in terms of code nor correct-looking in terms of result:

My questions (illustrated with arrows in the picture above):

1. Is this the right™ way to do it ?
2. I want the "dot filling" to go all the way to the right margin. Using two \dotfill commands somehow works, but the result doesn't look quite right (cf picture, there is extra space between the two dotfills). Is there a way out of this ?
3. How do I fix the wrong interline spacing below the coffin ? In the "central column" I'd like all lines to be equally spread apart.
4. Is there a more elegant way to horizontally join two coffins with adjacent text ? Here I'm joining bol and para with an explicit .5ex spacing and this feels wrong (even though it looks OK).

.

\documentclass{minimal}

\setlength{\parindent}{0pt}

\usepackage{xcoffins}

\begin{document}

\NewCoffin\cont% full-width container
\NewCoffin\para% multiline paragraph
\NewCoffin\bol % beginning of line 
\NewCoffin\eol % end of line

\SetVerticalCoffin\cont{\linewidth}{~}

\SetHorizontalCoffin\bol{In-coffin beginning of line.}

\SetVerticalCoffin\para{15 em}{Dummy text but long enough to span several lines at the specified width\dotfill}

\JoinCoffins \bol[B,r] \para[T,l](.5ex, 0pt)

\JoinCoffins \cont[B,l] \bol

\SetVerticalCoffin \eol {\CoffinWidth\cont - \CoffinWidth\bol}{\dotfill\rule{1ex}{1ex}}

\JoinCoffins \cont[\para-B,\para-r] \eol

A long chunk of text to show how the thing flows with surrounding lines. 
A long chunk of text to show how the thing flows with surrounding lines.
A long chunk of text to show how the thing flows with surrounding lines.
A long chunk of text to show how the thing flows with surrounding lines.

\TypesetCoffin\cont

A long chunk of text to show how the thing flows with surrounding lines. 
A long chunk of text to show how the thing flows with surrounding lines.
A long chunk of text to show how the thing flows with surrounding lines.
A long chunk of text to show how the thing flows with surrounding lines.


\end{document}

Answer 1

You can do it without coffins:

\documentclass{article}

\newenvironment{almostcenter}
 {\par\nobreak
  \leftskip=\dimexpr(\textwidth-15em)/2\relax
  \rightskip=\leftskip
  \parfillskip=-\rightskip
  \noindent\ignorespaces}
 {\unskip\dotfill\par\noindent\ignorespacesafterend}


\begin{document}

A long chunk of text to show how the thing flows with surrounding lines.
A long chunk of text to show how the thing flows with surrounding lines.
A long chunk of text to show how the thing flows with surrounding lines.
A long chunk of text to show how the thing flows with surrounding lines.
\begin{almostcenter}
Dummy text but long enough to span several lines at the specified width
\end{almostcenter}
A long chunk of text to show how the thing flows with surrounding lines.
A long chunk of text to show how the thing flows with surrounding lines.
A long chunk of text to show how the thing flows with surrounding lines.
A long chunk of text to show how the thing flows with surrounding lines.

\end{document}

Answer 2

Here are two minor adjustments to your MWE which produce the desired result.

The first adjustment is to nudge the \eol coffin a bit to the left when joining it to the \cont coffin. .22em is one half of the space which \dotfill uses for a box containing an individual dot. Tweak this as desired. Although the gap is less visible now, you might need a slightly larger adjustment for a perfect match.

\JoinCoffins \cont[\para-B,\para-r] \eol (-.22em,0pt)

The second adjustment involves adding \strut to the final line of \para. I have no idea why this is necessary as I don't know why the use of coffins (either the underlying expl3 functions or xcoffins) loses vertical spacing here.

\SetVerticalCoffin\para{15 em}{Dummy text but long enough to span several lines at the specified width \strut\dotfill}

Here's the result:

Complete code:

\documentclass{article}
\setlength{\parindent}{0pt}

\usepackage{xcoffins}

\begin{document}

\NewCoffin\cont% full-width container
\NewCoffin\para% multiline paragraph
\NewCoffin\bol % beginning of line
\NewCoffin\eol % end of line

\SetVerticalCoffin\cont{\linewidth}{~}

\SetHorizontalCoffin\bol{In-coffin beginning of line.}

\SetVerticalCoffin\para{15 em}{Dummy text but long enough to span several lines at the specified width \strut\dotfill}

\JoinCoffins \bol[B,r] \para[T,l](.5ex, 0pt)

\JoinCoffins \cont[B,l] \bol

\SetVerticalCoffin \eol {\CoffinWidth\cont - \CoffinWidth\bol}{\dotfill\rule{1ex}{1ex}}

\JoinCoffins \cont[\para-B,\para-r] \eol (-.22em,0pt)

A long chunk of text to show how the thing flows with surrounding lines.
A long chunk of text to show how the thing flows with surrounding lines.
A long chunk of text to show how the thing flows with surrounding lines.
A long chunk of text to show how the thing flows with surrounding lines.

\TypesetCoffin\cont

A long chunk of text to show how the thing flows with surrounding lines.
A long chunk of text to show how the thing flows with surrounding lines.
A long chunk of text to show how the thing flows with surrounding lines.
A long chunk of text to show how the thing flows with surrounding lines.


\end{document}