That's not how you calculate probability, the more minions in play the less chances Felmaw hits face. And no, Naralex is not rigged, from 5 dreams available, you get 2 and they can repeat, that's all.
Do you really understand how probability works?
You CAN rig probability.
Let's say there are 3 types of items, A, B, and C. If there is only one of each type, the chances of getting an A is only 1/3.
However, you CAN reduce the chances by adding more B or C items. For example, there is only 1 A, but there are 3 Bs and 3 Cs. The chances of getting A is now only 1/7.
These two RNGs are the same. You CAN rig them in the code. It's actually pretty easy.
In fact, Hearthstone has actually confirmed that they rigged some of the Discover card's RNG, so that they can't Discover itself.
Not discovering themselves is not rigging anything.. its simply removing a card from the pool of available randomness.