A reusable newlength (no stop/break if length already defined)

Your answer appears to work. A simpler method might consist of first \let-ting the length variable in question to \relax and then applying \newlength to it. Put differently, if you can't remember if a certain macro name has been used before to denote a length parameter (or anything else, really!) and if you're comfortable with (re)using it anyway, you can first \let it to \relax and then issue a \newlength directive.

\documentclass{article}
\begin{document}

\newlength\mytmplen
\setlength\mytmplen{10pt}
\the\mytmplen

\let\mytmplen\relax % let \mytmplen to \relax

\newlength\mytmplen % now it's safe to "re-issue" it as a length 
\setlength\mytmplen{20pt}
\the\mytmplen

\end{document}

Here is a \providelength command that will define a new length if not already defined, but also checks whether the command passed as argument has been defined with \newlength, in order to issue an error message if you try to use, say, \providelength{\textit}.

\documentclass{article}

\makeatletter
\newcommand{\providelength}[1]{%
  \@ifundefined{\expandafter\@gobble\string#1}
   {% if #1 is undefined, do \newlength
    \typeout{\string\providelength: making new length \string#1}%
    \newlength{#1}%
   }
   {% else check whether #1 is actually a length
    \sdaau@checkforlength{#1}%
   }%
}

\newcommand{\sdaau@checkforlength}[1]{%
  % get the first five characters from \meaning#1
  \edef\sdaau@temp{\expandafter\sdaau@getfive\meaning#1TTTTT$}%
  % compare with the string "\skip"
  \ifx\sdaau@temp\sdaau@skipstring
    \typeout{\string\providelength: \string#1 already a length}%
  \else
    \@latex@error
      {\string#1 illegal in \string\providelength}
      {\string#1 is defined, but not with \string\newlength}%
  \fi
}
\def\sdaau@getfive#1#2#3#4#5#6${#1#2#3#4#5}
\edef\sdaau@skipstring{\string\skip}
\makeatother

\begin{document}

\providelength{\foo}
\providelength{\foo}
\providelength{\textit}

\stop

Here is the terminal output:

\providelength: making new length \foo
\providelength: \foo already a length

! LaTeX Error: \textit illegal in \providelength.

See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
 ...                                              

l.31 \providelength{\textit}

? h
\textit is defined, but not with \newlength
? 

---

A version with expl3, that avoids the trick for finding if the command is already a length.

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\providelength}{m}
 {
  \cs_if_exist:NTF #1
   {
    \sdaau_providelength_check:N #1
   }
   {
    \msg_info:nnx { providelength } { making } { \token_to_str:N #1 }
    \newlength{#1}
   }
 }
\cs_new_protected:Nn \sdaau_providelength_check:N
 {
  \token_if_skip_register:NTF #1
   {
    \msg_info:nnx { providelength } { not-making } { \token_to_str:N #1 }
   }
   {
    \msg_error:nnx { providelength } { defined } { \token_to_str:N #1 }
   }
 }
\msg_new:nnn { providelength } { making }
 {
  \token_to_str:N \providelength :~making~new~length~#1
 }
\msg_new:nnn { providelength } { not-making }
 {
  \token_to_str:N \providelength :~#1~is~already~a~length
 }
\msg_new:nnnn { providelength } { defined }
 {
  #1~illegal~in~\token_to_str:N\providelength
 }
 {
  #1~is~defined,~but~not~with~\token_to_str:N\newlength
 }
\ExplSyntaxOff

\begin{document}

\providelength{\foo}
\providelength{\foo}
\providelength{\textit}

\stop

This is the excerpt from the log file

.................................................
. providelength info: "making"
. 
. \providelength: making new length \foo
.................................................
\foo=\skip49
.................................................
. providelength info: "not-making"
. 
. \providelength: \foo is already a length
.................................................


!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!
! providelength error: "defined"
! 
! \textit illegal in \providelength
! 
! See the providelength documentation for further information.
! 
! For immediate help type H <return>.
!...............................................  

l.47 \providelength{\textit}

? h
|'''''''''''''''''''''''''''''''''''''''''''''''
| \textit is defined, but not with \newlength
|...............................................