* Your investor is 100% right about not negotiating with blackmail. You should absolutely fire Andrew soon. Your relationship with him is unrecoverable. (The demand to do it "tomorrow morning" is itself close to blackmail, and you could postpone a little to collect your thoughts and make a less emotional decision. It is hypocritical for your investor to reject blackmail while blackmailing you.)
* Your investor (and you should own some of this responsibility too) has screwed you over by expecting software engineers to work at a startup with an eternally-postponed equity discussion. I don't even understand how that works. You may need to compromise with your investor by firing Andrew (which isn't really a compromise; it's the right idea even without your investor's demand) but also insisting on an equity discussion immediately so that you have something to show the rest of the team that you're listening and want to treat them right.
* Andrew's threat to take the team with him is confusing. Where does he think they'll go? They probably want jobs that pay them money, and it seems unlikely he has one of those ready tomorrow. He (and you) may be overestimating their attachment to him. If he behaves like this among them, they may even completely understand why he's leaving.
Seems like some combination of firing him, and having an honest conversation with the remaining team where you say that this is solely about him and not them, you want to make things right with the them regarding equity, and having stern words with your investor is the way to go?
(I have no qualifications which make this particularly good advice to follow, to be clear. But maybe simply being detached from your situation is helpful.)