I've been running it the way you suggest, but only allowing them to take a single stack of ammo when setting out. It's nice to know that someone agrees with me, I was wondering if how I ran it was too harsh. Thank you for your answers.
1. The stores sell harpoonbow shots, see the table on page 64. Of course, that requires the Heirs to find one and be able to communicate with the seller, which is not guaranteed.
I like your idea of letting the Heirs take as much ammo as they want when they set out. Only downside I could see is that they might be tempted to discard items that are not immediately obvious how they would be useful, such as the lens or tar pot. Has that happened to you?
2. The argument against is that ingesting Sand allows the Heirs to inherit memories of their ancestors. But, I agree that gameplay is more important here than flavour.
3. After making the mistake to allow people who had nothing to do during a rest to forage or explore, I agree that splitting turns seems too powerful.
But, regarding splitting turns, would you allow a split party to split their turns? For example if only half the group fail their morale roll to flee. I've yet had this come up, but I don't know how I would handle this.