So incapacitate her, and put her in a room that blocks/scrambles outgoing signals. Open her up, and have your team of six super geniuses pull her apart.
Still doesn't account for the fact that they could have come earlier, and as far as we know he didn't take serious precautions for anyone showing up anyway.
That's just not true. Oda made it clear from the reaction in the early stages of the broadcast that most people of the world believe him at his word, because he is that highly respected among the general populace.
If he was concerned with the WG twisting his words and explaining it away, then I don't know why he made it a specific point to not lay the blame on any side. He wrote his speech exactly in a way where the WG could easily explain it away, and just say some pirates stole the Ancient Weapon and used it. He clearly didn't care about that happening.
If he didn't want the WG to have the Mother Flame, why didn't he take any precautions to not let it fall into their hands after his death?
The spoilers say he wiped his memory only after two weeks, so he had to play pretend in that time. Again, really contrived, almost as if he had knowledge that the WG would just wait to kill him after he had time to create his broadcast rather than earlier, even though he's not supposed to have that knowledge.
So he just assumed someone would pick up the torch and not only read the books, at risk of their life, but also disseminate that knowledge, instead of being proactive and doing it himself. He has a unique capability to send information across the world unimpeded, but he just trusts that it will work out after his death? Nah, I think that's silly.