It always pays to take the worst case scenarios seriously. If you refuse to even consider them, when and if they turn out to be true, you find yourself utterly unprepared.

As to dead wood, as a cofounder and director this is his call too. Leaving the business may well be best for the business, and therefore best for him - so long as equity etc. is retained, although that's a dangerous position if you do not have or have only minority voting rights.

I am a 30 in a 30/30/40 company. I am terminally paranoid I'm due for the squeeze, so I plan for it and work against its possibility, as in my experience even your closest and best will apologetically drive in the knife, if the price is right. Pays to wear armour, in the format of contract law.