[Tex/LaTex] ntheorem with the [amsthm] option ignores styling of a theorem environment

Question

I'm using ntheorem with the amsthm option, to define a theorem-like environment for stating notes:

\documentclass{article}
\usepackage[amsthm]{ntheorem}
\theoremstyle{plain}
\theoremheaderfont{\normalfont\itshape}
\theorembodyfont{\normalfont}
\newtheorem*{myenv}{Note}

\begin{document}
\begin{myenv}
The quick brown fox jumps over the lazy dog.
\end{myenv}
\end{document}

I should be getting an environment beginning with the word 'Note' in italic font, followed by the contents in normal roman font. Instead, I get 'Note' in boldface roman and the body in italic:

Trying to define a new theoremstyle doesn't seem to work either:

\newtheoremstyle{nonumberabc}%
  {\item[\normalfont\itshape \hskip\labelsep ##1\theorem@separator]\normalfont}%
  {\item[{\normalfont\itshape \hskip \labelsep ##3}\theorem@separator]\normalfont}
\theoremstyle{abc}
\newtheorem*{myenv}{Note}

this gives the same results with the above document.

What am I doing wrong?

The problem does not occur if I drop the amsthm option – but then I don't get some nice amsthm definitions I'm expecting… (it was psychologically hard enough to drop amsthm for ntheorem after years or use!) Most importantly, I like the amsthm proof environment. So, for those suggesting that I just drop amsthm – how can I reproduce proof with the exact same behavior (including the QED box size etc.) with ntheorem? I can't just lift code out of ntheorem, because it doesn't seem to use its own theoremstyle commands to get the proof environment. Rather, it does something else.

Answer 1

It is possible to "lift" the proof environment code from ntheorem:

\documentclass{article}
\usepackage[thmmarks]{ntheorem}% http://ctan.org/pkg/ntheorem

\makeatletter
\newcommand{\openbox}{\leavevmode
  \hbox to.77778em{%
  \hfil\vrule
  \vbox to.675em{\hrule width.6em\vfil\hrule}%
  \vrule\hfil}}
\gdef\proofSymbol{\openbox}
\newcommand{\proofname}{Proof.}
\newcounter{proof}\newcounter{currproofctr}\newcounter{endproofctr}%
\newenvironment{proof}[1][\proofname]{
  \th@nonumberplain
  \def\theorem@headerfont{\itshape}%
  \normalfont
  %\theoremsymbol{\ensuremath{_\blacksquare}}
  \@thm{proof}{proof}{#1}}%
  {\@endtheorem}
\makeatother

\theoremstyle{plain}
\theoremheaderfont{\normalfont\itshape}
\theorembodyfont{\normalfont}
\newtheorem*{myenv}{Note}

\begin{document}
\begin{myenv}
The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog.
\begin{proof}
This statement is true, because the fox did jump over the lazy dog.
\end{proof}
\end{myenv}
\end{document}

The QED-symbol is displayed when using the thmmarks package option of ntheorem. In ntheorem this is manually defined in \openbox, and subsequently used as \proofSymbol.