At the risk of stating the obvious:
1. People can not stay off the grid for long with no money. If they voluntarily disappeared, they took cash. Check bank activity.
2. A woman who enrolls her child in a private school values education. I can't imagine her keeping her daughter out of school for too long.
3. A responsible woman with an elderly mother wouldn't want to worry her.
4. Someone who was deliberately planning to disappear long-term would have made arrangements. By arrangements in this case, I mean possibly taking passports, and making sure that the farm was in good hands.
Those are my top four. This is a great mystery case, and I don't think we have enough clues at the present time. Maybe Iona was desperately in need of some time off and they extended their planned vacation for a bit longer. BUT, if that were the case, you'd think she'd charge her phone and check in with her mom.
Search her computer for recent activity and search her house for their passports.