"Must" is indeed sometimes used to mark probability in certain informal contexts. Examples:
Given how well the Netherlands has been playing, they must surely win the championship this year.
He has many umbrellas. He must have a hard time picking one.
But it's not a good usage if there is the least possibility of confusion with the usual "must" of obligation or necessity. Therefore your second sentence is perhaps acceptable, although it should be avoided in formal writing. Your first sentence is very questionable, I think.