What do the rules and language of the oath suggest about the boys, especially Tom Sawyer who composed it?

The rules and the language are reminiscent of old prate stories..... things the boys had heard and repeated. From the text, we can infer that they were superstitious and had fabulous imaginations. It also suggests their fear.

“Huck Finn and Tom Sawyer swears they will keep mum about This and They wish They may Drop down dead in Their Tracks if They ever Tell and Rot.”

Huckleberry was filled with admiration of Tom’s facility in writing, and the sublimity of his language. He at once took a pin from his lapel and was going to prick his flesh, but Tom said:

“Hold on! Don’t do that. A pin’s brass. It might have verdigrease on it.”

“What’s verdigrease?”

“It’s p’ison. That’s what it is. You just swaller some of it once—you’ll see.”

So Tom unwound the thread from one of his needles, and each boy pricked the ball of his thumb and squeezed out a drop of blood. In time, after many squeezes, Tom managed to sign his initials, using the ball of his little finger for a pen. Then he showed Huckleberry how to make an H and an F, and the oath was complete. They buried the shingle close to the wall, with some dismal ceremonies and incantations, and the fetters that bound their tongues were considered to be locked and the key thrown away.