1. My players did so, with the hermits. So far, they haven't had many empty slots to fill, and have often been forced to leave something behind before setting out. 11 slots fill up quickly with the heirloom, general starting equipment, and the trade equipment.
2. A Rasp of Sand is my (and my players') first old-school module/system, and I don't have a good feeling for balance yet. So I'm cautious about deciding that something works in a way that's disadvantageous to the Heirs. I don't know how this particular ruling is working, as the fisher decided to change to magician soon after we encountered the issue.
3. I've had the group failed their morale rolls by differing amounts a couple of times (and thus being split), but they've been able to find each other again quickly. What's worked for me so far is to let them all declare what they're doing, and then we roll and narrate everything.
It's been a fun guessing game for the players to try to figure out where they are in relation to what they've already explored. Also, them knowing that the generation is over if anyone of them falls also adds some investment into what everyone is doing even when their Heir isn't present.
After some thought, why not allow the Heirs to split turns, but roll an encounter for each action they choose to do? Not splitting turns would be safer, but doing so allows them to do things faster - not usually relevant, but might be if an Heir is poisoned or such. This would also be easy to adapt to a split party - roll one encounter for each location.