I think that, even at the low end, a 15% rate of maternal mortality per birth is probably too high for a society to sustain. The historical pre-modern rate is assumed to be around 1-1.5% (estimates are rough, given the poor quality of the data). So you're talking about a maternal death rate a full order of magnitude higher than known in actual humans. If you retain historically high levels of infant mortality with this - and you should, in fact infant mortality rates would almost certainly increase, since there would be fewer mothers available to care for young children.

If your society needs 4 children per woman to reach replacement rate in the population (due to infant mortality, disease, war, etc.), you're going to have some issues. First, if you need 4 children per adult woman, you actually need more than 4 per fertile adult woman because probably around 10% of your female population is infertile to begin with and cannot have any children.

So your child bearing pool goes like this:
Birth One: 0.9 X 0.85 = 0.765 - portion of the female population available for birth 2
Birth Two: 0.765 X 0.85 = 0.65 - available for birth 3
Birth Three: 0.65 X 0.85 = 0.55 - available for birth 4
Birth Four: 0.55 X 0.85 = 0.47 - available for birth 5
Birth Five: 0.47 X 0.85 = 0.40 - available for birth 6
Birth Six: 0.4 X 0.85 = 0.34 - available for further births

So, 6 births in with 15% your female population is down to 44% of the initial cohort size, and of the remainder nearly a third are the infertile woman who can't conceive and have avoided this whole process. Out of the fertile portion of the population nearly 2 in 3 will die just working to keep the population stable over time. That might be survivable, but it's frightfully brutal. Society would probably be forced to identify fertile woman shortly after puberty and enforce pregnancies upon them. Infertile women would acquire a massive amount of power in this society, because fertile women will be heavily controlled and most will die before they turn 30. Essentially, you're producing The Handmaid's Tale, only worse and without any possibility for improvement since if women don't go through this Russian Roulette game of childbirth the population collapses.

If you crank the mother mortality any further it gets impossible pretty fast. At 25% you're down to 50% after 2 births and 15% after 6. That's not enough to be sustainable, the population would die off.