otf sans serif math only in figures

Question

I was going to use a serif font (TG Pagella) for most of the text (including captions) and a sans serif font (Candara) for my (tikz) figures. For text this works nice, but not for maths. First I used only unicode-math with the version key to switch to the sans version via \mathversion{...}. The problem is that unicode-math expects an otf math font which does not exists for Candara. Using the normal Candara font results in up-right math, but I want it to be italic. With the italic font everything in math becomes italic even numbers.

So next I tried to use mathspecs. There the sans math works nice. The problem is that I don't know how to switch between sans and serif, because there is no version option for \setmathsfont and \setmathsfont can only be called from the preamble. So I only used the version key for Pagella and tried to switch to the sans font via \mathversion{normal} but this does not really work. Some how the math font is then always Candara (see below).

Is there any way to get that work?

\documentclass[a5paper]{memoir}

\usepackage{tikz}
\usepackage{pgfplots}
\usepgfplotslibrary{units}
\pgfplotsset{compat=1.8}

\usepackage{mathspec}

\usepackage{unicode-math}
\setmainfont[Numbers=OldStyle]{TeX Gyre Pagella}
\setmathfont[version=tgp]{TeX Gyre Pagella Math}
\setsansfont[Scale=MatchUppercase]{Candara}
\setmathsfont(Digits,Latin,Greek)[Scale=MatchUppercase]{Candara}

\usepackage{siunitx}
\SendSettingsToPgf

\tikzstyle{every picture}+=[font=\small\sffamily,
                execute at begin picture=\mathversion{normal}
                ]
\pgfplotsset{unit code/.code 2 args={
                    \sisetup{detect-family}
                    \si{#1#2}}}
\pgfplotsset{general plot/.style={ 
        xtick pos=left,
        ytick pos=left,
        enlarge x limits=false,
        minor x tick num=1,
        every x tick/.style={color=black, thin},
        every y tick/.style={color=black, thin},
        tick align=outside,
        xlabel near ticks,
        ylabel near ticks,
        unit markings=parenthesis
    } 
}

\usepackage{blindtext}
\parindent=0pt
\mathversion{tgp}
\begin{document}
    $V_{\textup{DS}}$ in \si{V}\\
    1234567890\\
    \textsf{1234567890}
    \begin{figure}
        \begin{tikzpicture}
            \begin{axis}[
                general plot,
                xlabel=Voltage $V_{\text{DS}}$,
                x unit=\volt,
                change y base=true,
                y SI prefix=micro,
                ylabel=Current $I$,
                y unit=\ampere]
            \addplot {1.3E-6*x};
            \end{axis}
        \end{tikzpicture}
        \caption{The current $I$ in \si{\micro\ampere} versus the voltage $V_\textup{DS}$ in \si{\volt}}
    \end{figure}
\end{document}

Edit: Following the comments of Khaled Hosny I tried to use only unicode-math. There was one problem the range option does not work for version-tagged fonts. So I made two commands to switch between the settings. And indeed this works fine for math in the body. But if I use them in conjunction with the tikz key execute at begin picture I get building errors:

I can only handle fonts at positive sizes that are
less than 2048pt, so I've changed what you said to 10pt.

LaTeX Font Info:    Font shape `EU1/CandaraItalic(0)/m/n' will be
(Font)              scaled to size 0.0pt on input line 57.
! Improper `at' size (0.0pt), replaced by 10pt.
<to be read again> 
               relax 
l.57 ^^I^^I^^I\end{axis}

One other error message is:

*************************************************
* fontspec warning: "script-not-exist-latn"
* 
* Font 'Candara' does not contain script 'Math'.
* 'Latin' script used instead.
*************************************************
! Font \nullfont has only 7 fontdimen parameters.
<recently read> \font 

And here the source code for that:

\documentclass[a5paper]{memoir}

\usepackage{tikz}
\usepackage{pgfplots}
\usepgfplotslibrary{units}
\pgfplotsset{compat=1.8}

\usepackage{unicode-math}
\setmainfont{TeX Gyre Pagella}
\newcommand{\rmmath}{\setmathfont{TeX Gyre Pagella Math}}
\rmmath
\setsansfont[Scale=MatchUppercase]{Candara}

\newcommand{\sfmath}{
    \setmathfont[Numbers=Lining, Scale=MatchUppercase]{Candara}
    \setmathfont[range=\mathit, Scale=MatchUppercase]{Candara Italic}
}

\usepackage{siunitx}
\sisetup{detect-family}
\SendSettingsToPgf

\tikzstyle{every picture}+=[font=\small\sffamily,
% This does not compile:
                execute at begin picture=\sfmath,
                execute at end picture=\rmmath
                ]
\pgfplotsset{unit code/.code 2 args={
                    \si{#1#2}}}
\pgfplotsset{general plot/.style={ 
        xtick pos=left,
        ytick pos=left,
        enlarge x limits=false,
        minor x tick num=1,
        every x tick/.style={color=black, thin},
        every y tick/.style={color=black, thin},
        tick align=outside,
        xlabel near ticks,
        ylabel near ticks,
        unit markings=parenthesis
    } 
}

\parindent=0pt

\begin{document}
    \begin{figure}
        \begin{tikzpicture}
            \begin{axis}[
                general plot,
                xlabel=Voltage $V_{\text{DS}}$,
                x unit=\volt,
                change y base=true,
                y SI prefix=micro,
                ylabel=Current $I$,
                y unit=\ampere]
            \addplot {1.3E-6*x};
            \end{axis}
        \end{tikzpicture}
        \caption{The current $I$ in \si{\micro\ampere} versus the voltage $V_\textup{DS}$ in \si{\volt}}
    \end{figure}
    {\sffamily V    \sfmath $5 V_{\mathrm{DS}}$ in \si{V}}\\
    V \rmmath $5 V_{\textup{DS}}$ in \si{V}\\
\end{document}