Answers to questions as numbered:
1. There's no time limit for an executor to distribute an estate. The executor doesn't have to pay a penny to any of the grandchildren until everything is sorted out, including the sale of the house.
2. The executor is already legally bound, by the terms of the will and of his oath, to distribute the estate in accordance with the provisions of the will.
3. If the executor moves abroad, taking all the money with him, you'll have a massive battle to get the money back. He'd be guilty of a criminal offence but it's non-extraditable, so your only way of taking action against him would be through the foreign courts.
4. If the second great grandchild is not named in the will (or in a codicil thereto), the executor must give �10,000 to the first great grandchild and nothing at all to the child who was born later. (I know that seems unfair but the executor is bound by the provisions of the Wills Act 1837).
Chris