Quickly reload nix expression in nix-shell?

No other simple options, sorry. I can only think of rewriting nix-shell to accomplish what you want. Probably it's not even that hard. Reparse the expression, cleanup the env and refill the env.

New project lorri is built exactly to support this feature. It requires shell.nix file to be present and depends on direnv.

https://github.com/target/lorri