@xellga, thank you for the kind words and for taking the time to articulate so well what you enjoyed and your questions! I’ll do my best to answer.
Mullow is in part testament to the fact that Theurges don’t design things entirely from scratch, but amend the natures of existing things. The plant with the closest properties to what they wanted was a fast-spreading weed, and changing that aspect of its nature to make it harder to cultivate would have been one more challenge and cost.
That said, the aristocratic Theurges who created mullow weren’t troubled by the idea that it would be widely available. They saw society as a raft, where the free classes were kept “afloat” by the sacrifice of the unfree classes. This would work best, they thought, with as few of the former and as many of the latter as possible.
The preferential Harrowing of childless helots creates a strong disincentive for their mullow use (while they’re also fed shrub yams, which secretly have the opposite effect). Meanwhile, the fact that everyone else can access mullow easily will keep the numbers of free folk manageable.
There have no doubt been subsequent Theurges who questioned the wisdom of this analysis. But once the altered mullow was released into the wild, there was no going back.
As for religious dogma, the fact that mullow makes it easier to pretend that the upper classes are living up to the chastity ideal is a bonus. Halassur is pro-natalist and has a strong religious aversion to contraception; the Hegemony does not.
I agree, but I’m barely qualified to play with it even on that level. 
Let me chew on the idea and get back to you!
It is risky, and we’ll see whether I still feel that way when I’m wrapping up the later games. I reserve the right to change my mind and have a big explicit reveal. But we often have to make the most important decisions in life based on incomplete and ambiguous evidence, and I’d like the Breden plot to reflect that, even after multiple playthroughs.
Thanks for expressing so well what I was trying to convey with that dynamic–and I’m glad you liked it!