the statements taken as a whole are true

You mean that all the statements are true -- right? You're evaluating "a AND b AND c AND d AND e AND f"?

The P of each statement must then lie between 0.7 and 1.0

Correct.

if they are equal then the P of each statement is 0.7 ^ (1/6) = 0.94

You are assuming the statements are independent of each other. That's not necessarily so.

To take an extreme example, all six statements could be a function of the same single property/event. In such a case the P of each is 0.7 and the P of all of them is still 0.7.