This is one example (of many) of an 18th Dynasty chest, c. 1370 BCE, located in a museum near my home.
I would love to see a demonstration of how the Ark could be transformed into a dangerous machine, I've heard it claimed many times but I don't buy it. I have a biscuit tin that is also an electrical conductor, yet no-one has been hurt - the biscuits, like the pot of manna, have mysteriously disappeared though. So, I see no reason to believe that the Ark was anything other than an elaborately decorated chest that held precious artifacts. I'm certainly not saying there wasn't a dangerous device, just that the Ark is not it.
The Knights Templar do seem to be involved, and I'd argue that the Grail also is tied up in all this, with a great deal of activity and new material occurring in the latter half of the 12th century.