The behavior of quotes can and should be described in terms of how they affect the words they expand to (i.e. the argv you build), so yes, it clarifies all this.

For example, all the various wrong ways of quoting var="My File.txt" or otherwise incorrectly using such a name will result in variations on a wrong argument list:

  execlp("cat", "$var", NULL);            // cat '$var'
  execlp("cat", "My", "File.txt", NULL);  // cat $var
  execlp("cat My File.txt", NULL);        // cmd="cat $var"; "$cmd"
  execlp("cat", "'My", "File.txt'", NULL);// cmd="cat '$var'"; $cmd
  execlp("cat", "My\\", "File.txt", NULL);// cmd="cat My\ File.txt"; $cmd
  execlp("cat", "'My File.txt'", NULL);   // var="'$var'"; cat "$var"
  execlp("cat", "\"$var\"", NULL);        // arg='"$var"'; cat $arg

Meanwhile, all the correct ones result in the same, correct argv:

  execlp("cat", "My File.txt", NULL);  // cat "$var"
  execlp("cat", "My File.txt", NULL);  // cat 'My File.txt'
  execlp("cat", "My File.txt", NULL);  // cat "My File.txt"
  execlp("cat", "My File.txt", NULL);  // cat My\ File.txt
  execlp("cat", "My File.txt", NULL);  // cmd=("cat" "$var"); "${cmd[@]}"
  execlp("cat", "My File.txt", NULL);  // arg="'My File.txt'"; eval "cat $arg"

If you don't know which argument list you're aiming for, you basically have to go by guesswork and superstitions.