Still having difficulty with use of `\expandafter`

I'm a bit confused by your code. But there are a couple of things I see that might help you.

Writing

\expandfater\getlength{#1}

is effectively the same as writing

\getlength{#1}

without any expansion.

If it's #1 that you want expanded first, that's not going to happen as you wrote it. Instead, it's the { TeX is going to try to expand. To reach #1 you need to write something like

\expandafter\getlength\expandafter{#1}

But this most likely won't do what you want either. If #1 is a string of tokens, for example if you try

\def\a{ABC}
\def\b{\c}
\def\c{XYZ}
\def\mythe#1{\expandafter\getlength\expandafter{#1}}

Then

\mythe{\a\b\c}

expands to

\expandafter\getlength\expandafter{\a\b\c}

which then expands to

\getlength{ABC\b\c}

The \b\c of #1 is not accessible through the use of \expandafter as you've written it. Now, if you know that #1 should only be one token, that all is OK at this point.

If you want to get the integer and fractional parts of a dimension, then the following will accomplish that without calling any special packages.

\documentclass{article}
\makeatletter

\def\getparts#1{%%
  \edef\my@stripped@length{\strip@pt#1}%%
  \expandafter\ae@int@frac\my@stripped@length..\@nil
}

\def\ae@int@frac#1.#2.#3\@nil{%%
  \def\aeinteger{#1}%%
  \def\aefraction{#2}%%
}

\newlength{\aetemp}
\setlength{\aetemp}{1.234cm}

\def\showparts{%%
  \begin{tabular}{ll}\hline
  Length   & \the\aetemp\\
  Integer  & \aeinteger \\
  Fraction & \aefraction\\\hline
  \end{tabular}}


\makeatother
\pagestyle{empty}
\begin{document}

\setlength{\aetemp}{1cm}
\getparts{\aetemp}
\showparts

\setlength{\aetemp}{215pt}
\getparts{\aetemp}
\showparts

\end{document}

The problem is that \getlength requires several expansion steps to end its job delivering a sequence of digits (with a decimal dot in the middle). I'll show them in successive lines

\getlength{\thislength}
\strip@pt\thislength
\expandafter\rem@pt\the\thislength
\[email protected]
123\ifnum 456>\z@ .456\fi
123.456

A total of five expansion steps that require 31 \expandafter tokens to be performed if you want that \@printplainbefore to see what it expects, not just one. But the presence of \ifnum makes the macro unusable in practice.

Since you don't want to remove a zero fractional part, you can use a \romannumeral trick:

\documentclass{article}

\makeatletter
\def\printplainbefore#1{\expandafter\@printplainbefore\romannumeral-`Q#1..\@nil}
\def\@printplainbefore#1.#2.#3\@nil{#1}
\def\printplainafter#1{\expandafter\@printplainafter\romannumeral-`Q#1..\@nil}
\def\@printplainafter#1.#2.#3\@nil{#2}

\begingroup\catcode`P=12 \catcode`T=12
\lowercase{\endgroup\def\simplerem@pt#1PT{#1}}
\def\simplestrip@pt{\expandafter\simplerem@pt\the}
\newcommand*{\getlength}[1]{\simplestrip@pt#1}

\makeatother

\newlength{\thislength}
\setlength{\thislength}{123.456pt}

\begin{document}
Number: 123.456
    \quad Split: \printplainbefore{123.456} -- \printplainafter{123.456}

\def\temp{123.456}
Macro: \texttt{\meaning\temp}
    \quad Split: \printplainbefore{\temp} -- \printplainafter{\temp}

The: \the\thislength
    \quad Split: \printplainbefore{\the\thislength} -- \printplainafter{\the\thislength}

Getlength: \getlength{\thislength}
    \quad Split: \printplainbefore{\getlength{\thislength}} -- \printplainafter{\getlength{\thislength}}

\end{document}