The Unseen can roll Prestige classes. Here’s how I roll slots.
I basically assign a value to each class type in between one to five and I roll it as many times as there are random HoB.
Each time a roll hit, say I roll 3 times 1, which means 3 investigatives in the match.
Then I assign a value for each of these slots by rolling a dice from one to the maximum number of class types for the role, such as for investigatives if I roll a 2, 2, and 5 then that’s two Fortunetellers and a Watchman.
Maximum of 4 of each class and if the number exceeds that I reroll the slot completely.
I do this until all Random HoB slots are filled and for the Random Unseen I roll a one to whatever many possible Unseen coverts could roll, as in no Blasphemer nor Pardoner. If something arises like a 4th Prestige slot or 4 Surgeons and a Plague Doctor then I reroll it.
Then I roll 2d4 for the Cult and fill in the slots depending on the rolls, such as 1,3 being Apostle and Ritualist, 2,2 being two Invokers, and so on.
So in short, I roll HoB first, then if less than three Prestige classes were rolled then it’s possible that the Unseen Prestige classes rolled.
The starting Apostle can only take abilities from the Cabalist though due to the Unique restriction of Cult Alts.