Trying to identify a mystery-ish book?

It had 2 young protagonists, possibly in their teenages, one was a girl.

The antagonist (or what seemed to be the antagonist, i forgot), was a man in a suit that gave them trials or puzzles.

But it could also be like that he didn't really give them trials or puzzles and rather they walked into the next ones themselves to solve something?

the book cover was purplish and the name of the book was long.

I think there was a segment where they almost died, possibly from drowning or suffocation before solving it last second?

There was a riddle where the answer of it was quite literally just a letter, it went something like,

I'm always infront of Ants.

Always behind Lava.

And always within May.

What Am I?

And the answer was the letter 'A'.

There was supposedly a part where the cast had to dance to solve a puzzle? (Or had to physically step on things to solve it.)

It was placed in atleast a two story setting, because there was a staircase. It was either a mansion or a family owned shop/house.

Please help!