hhline drawn over vertical lines in a table

This was a very challenging one. I have simplified your code, removed seemingly unnecessary multirow and multicolumn. The reason for you problem was the ?-column, because hhline did not recognise it as a column, so it was not possible to ‘cross’ it the correct way.

Here is the result and the code:

documentclass[a4paper]{article}
usepackage{multirow,hhline}
usepackage[table]{xcolor}
usepackage[margin=2cm]{geometry}

renewcommand{arraystretch}{1.2}
setlength{extrarowheight}{1pt}

begin{document}

% Define colours and columns
definecolor{Purple}{RGB}{101, 60, 139}

newcolumntype{L}[1]{>{raggedrightletnewlinearraybackslashhspace{0pt}}p{#1}}
newcolumntype{C}[1]{>{centeringletnewlinearraybackslashhspace{0pt}}p{#1}}

arrayrulecolor{white}
setlengtharrayrulewidth{0.5mm}


begin{tabular}{L{5cm}||c|c|c||C{1cm}||C{1cm}||C{1cm}}
    hline
    % R1C1
    cellcolor{Purple} & 
    % R1C2-4
    multicolumn{3}{c||}{cellcolor{Purple}textcolor{white}{Combined cell heading}} &  
    % R1C5
    cellcolor{Purple} & 
    % R1C6 
    cellcolor{Purple} & 
    % R1C7
    cellcolor{Purple}
    % Drawing line to split first row
    hhline{>{arrayrulecolor{Purple}}->{arrayrulecolor{white}}||---||>{arrayrulecolor{Purple}}->{arrayrulecolor{white}}||>{arrayrulecolor{Purple}}->{arrayrulecolor{white}}||>{arrayrulecolor{Purple}}-}
    % R2C1
    cellcolor{Purple}textcolor{white}{A} & 
    % R2C2
    cellcolor{Purple}textcolor{white}{B} & 
    % R2C3
    cellcolor{Purple}textcolor{white}{C} & 
    % R2C4
    cellcolor{Purple}textcolor{white}{D} & 
    % R2C5
    cellcolor{Purple}textcolor{white}{E} & 
    % R2C6
    cellcolor{Purple}textcolor{white}{F} &
    % R2C7
    cellcolor{Purple}textcolor{white}{G}
end{tabular}


end{document}

With {NiceTabular} of nicematrix (with the latest version : 5.2 2020-08-19), you have directly the expected result (because cline is respected by the coloring tools provided by {NiceTabular}).

documentclass{article}

usepackage{nicematrix}
usepackage{xcolor}

begin{document}

arrayrulecolor{white}
doublerulesepcolor{white}

begin{NiceTabular}{wl{5cm}||wc{1cm}|wc{1cm}|wc{1cm}||wl{1cm}||wl{1cm}||wl{1cm}}%
 [ code-before = rowcolor[RGB]{101, 60, 139}{1,2} ]
& Block{1-3}{color{white}Combined cell heading} 
cline{2-4}
color{white}A &color{white} B &color{white} C &color{white} D &color{white} E &color{white} F &color{white} G 
end{NiceTabular}

end{document}