Nesting environment inside new command generating errors

Extreme TeX novice, sorry for the length of the example, I wasn’t sure what packages were important to replicate the problems. I was trying to create a simple command that I could use to injected labeled comments into an existing document. My comment would be in text color blue and be prefixed with initials (‘[XXX]’ in this example)

documentclass[12pt]{article}
usepackage{cprotect}
usepackage{amssymb, amsmath, bm, bbm}
usepackage{adjustbox}

usepackage{amsmath, amsfonts}
usepackage{bm, bbm}
usepackage{graphics, graphicx, color, epsf}
usepackage{natbib}
usepackage{xr, zref, hyperref}
usepackage{etoolbox, siunitx}
%
usepackage{xcolor}
%
usepackage{scrextend}
%
newcommandxxx[1] {newlinetextcolor{blue}{[XXX] #1}}

begin{document}
title{A title}
author{A person}
maketitle
section{A section}
Some section stuff
xxx{a comment}
begin{verbatim}
This is 
text that 
presented 'as-is'
end{verbatim}
Some more text
xxx{
abc
begin{verbatim}
verbatim
end{verbatim}
}
end{document}

Generates:

line 37: Argument of @xverbatim has an extra }. } 
line 37: Paragraph ended before @xverbatim was complete. } 
line 37: Extra }, or forgotten endgroup. } 
: Emergency stop. 
: Command @footnotemark has changed.

I tried to define a newenvironment (not shown) used by the command, thinking that perhaps the ‘command’ couldn’t handle nested content, but it didn’t help.

1. I’d like to understand what I have done wrong and how I might figure out such issues in the future.

2. Find a concise summary of the TeX syntax (not LaTeX, just basic basic TeX).

3. Understand how to get rid of that footnotemark warning. Is some package redefining that command?

documentclass[12pt]{article}
usepackage{xparse}
%
usepackage{xcolor}
%
begingroup
newcommandxxx[1]{%
  endgroup
  newcommandxxx{%
    begingroup
    catcode`^^I=12 %
    innerxxx
  }%
  NewDocumentCommand{innerxxx}{+v}{%
    endgroup
    textcolor{blue}{[XXX] %
    begingroup
    newlinechar=endlinechar
    scantokens{endgroup##1#1}}%
  }%
}%
catcode`%=12relax
xxx{%}%

begin{document}

title{A title}
author{A person}

maketitle

section{A section}

Some section stuff

xxx{a comment}bla

begin{verbatim}
This is 
text that 
presented 'as-is'
end{verbatim}

Some more text

xxx{%
abc
begin{verbatim*}
verbatim
end{verbatim*}
}%

Indenting is funny when nesting verbatim-environment in verbatim-argument:

xxx{%
  abc
  begin{verbatim*}
  verbatim
  end{verbatim*}
}%

Indenting is funny when nesting verbatim-environment in verbatim-argument:

xxx{%
  abc
  begin{verbatim*}
  verbatim
    verbatim
end{verbatim*}
}%

end{document}