Don't worry - there are other opportunities for your young villagers to learn construction skills. Unfortunately, a villager must have at least some construction skill in order to initiate building a new hut, so you'll have to wait until you can train one of your remaining villagers in construction (you should have at least two puzzles left that would work for that). There is an Island Event that will also help you.
There are a number of reasons for a 'missing' tombstone, but the most common is this: If a villager gets 'interrupted' while burying a corpse, either by you or another villager, they will 'drop' the remains and no grave will be created.
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Barbara
Unicorn
Last Day of Work