About the first thing, indeed it's possible to git commit --amend --author=<info@asd.com> Mrs. Foo" and it works and I suggest it to restore the original credit. I've done it a couple of times and it worked for me.

About the second thing, I think you are right. If I understand correctly, you are saying that, if you try to use this in the commit message, it does not work:

Signed-off-by: ...

It may not work since arc diff thinks it's a section. Right?

In case, that is a bug! \o/ Feel free to file a Task under Arcanist.

As workaround, while waiting for a fix, maybe you can, whoops, forget the colon, and then it's safe for you to pretend to have forgotten it by mistake. Example:

Signed-off-by @l2dy

I will sincerely trust you, and believe me, I think somebody will land that even if the syntax is not the canonical one. But indeed we will go straight to the hell for that. But I don't care of hell. So, feel free to do it.