Factual error: A 250-year-old sea chest recovered from the ocean floor is opened, and all its paper items are perfectly dry: the log book can be opened and read, and the letters can easily be unrolled. Highly unlikely. The chest's wood and pitch would deteriorate (salt water is extremely corrosive), and the paper should be damp and crumbly, at the very least. Even more likely, it would have become a lump of unreadable sludge.
Factual error: Some factual errors regarding Basques: 1) Basques have not been fighting for ages against France and Spain, and they do not kidnap US citizens. 2) Basques do not look, in any way, like South Americans. 3) Violent Basques are not guys "up on a mountain" with old rifles, but terrorists with modern weapons.
Factual error: MacGyver and Victoria go back to the location of the science event to think about the incident. MacGyver's friend, Bannister, suffers from a heart attack. MacGyver cuts a microphone cable and binds the cords to a metallic candlestick. MacGyver uses his creation as a some sort of defibrillator. Of course it works and MacGyver saves his friend. The maximum power in the wire is 11-48v DC with the maximum current of 10MA - not enough for giving huge electric shocks.
Answer: Late one night, Geoffrey used a covert listening device to overhear David explaining his barricade device to Janet which is how he figured it out.
How do we know Geoffrey really did use a covert listening device? They didn't mention him using one.
There was a scene of him using it when David told Janet about his barricade.