https://github.com/nix-community/nix-doom-emacs - declarative

https://github.com/hlissner/dotfiles/blob/master/modules/editors/emacs.nix - imperative (used by the doom emacs author, this is probably more hackable)

privacy.resistFingerprinting in about:config forbids the websites to use canvas by default among other things, you can override it using site permissions

Try a compiler flag -I$HOME/.nix-profile/include, or use a shell.nix with pkgs.mkShell

#!/usr/bin/env nix-shell
#!nix-shell -i bash /path/to/shell.nix
echo "Hello, World!"

haven’t tested, but it should work

Houdini is available in nixpkgs, it just isn’t in the binary cache and requires you to download the necessary files (add it via nix-store --add-fixed sha256 ./houdini-19.5.569-linux_x86_64_gcc9.3.tar.gz). Nuke isn’t, but you can look at the Houdini derivation for reference.

Overlays don’t “provide any options”. Overlays simply extend nixpkgs or replace parts of it. You can search through all packages in nixpkgs here.