I can confirm this with the latest vim-nightly.
your vimrc loads first (where you have defined your highlighing)
then syntax/synload.vim does override it with:
" Set the default highlighting colors. Use a color scheme if specified.
exe "colors " . colors_name
where syntax/syncolor.vim ...
My answer is very similar to Martin Tournoij (full credit to him).
This just makes it in a single-line command. When you run this, it will disable continuation of comment permanently:
echo 'au FileType * set fo-=c fo-=r fo-=o' >> ~/.vimrc
Ok, so I have a prototype of a license concealer (not fold) written, but I think because of the way conceal and cchar work, I can't get the lines to collapse into one. For that, folding may be better.
A quick example, that doesn't work with comments (yet):
let text =<< trim END
Copyright (c) 2016 David Ben Knoble
Permission is hereby ...
You can call winwidth() to get the number of chars in line that fit the screen.
And use printf('%#'. width. 's' , str). It should fill it with #, in a similar way to zero fill (I believe).
You can encapsulate it in a function to be called by a command.