Dunno if you're still looking for character survivability rates, but I'm gonna throw in my so far:
Playing as Cornovia, I've reached the year 1108, and am on my third king.
The story so far:
My starting king Magnus I, had 2 daughters and 1 son by the age of 25. Then he was assaulted by a brutal murderer (event thingy about murderer stalking your town, 50/50 chance of winning, or getting some serious negative traits i didn't need, had already had to swallow 3 negatives to avoid getting stressed), and turned incapable. Lived to the age of 40 before he got drowned by the people bathing him.
His son, Magnus II succeeded him, he likewise had 2 daughters and one son, before dying at the age of 26 of severe stress. (it was the 4th or so event with stress as the potential outcome, I must admit i think there's too many of them given the severe restrictions the human player get, he started out with decent stats, but all the negatives traits were really starting to hurt). He died the day after getting stressed...
on my third ruler now, Magnus III age 17, no children so far, but why is pregnant.
To sum up: I don't notice any real problem with fertility. I've had plenty of events to give me the "lustful" traits to get things going even faster, I've not had that many sons, but that's related to my rulers dying young due to decisions I made.
However, I must admit that i think there's either too many stress related events, or they fire too often. In most of them there's either a 50/50 chance of you getting stressed, or a 100 % chance to get a negative trait. This wouldn't neccesarily be bad, but by the time my 2 first rulers had turned 25 they had both had atleast 3 stressful events happen. I think that's just a tad too much for a 1 province king tbh. It wouldn't be all that bad, but the base stats gain is just too low. I was just hovering above having the "corruption" modifier hit me again, while still dealing with diplomatic catastrophy & other modifiers.. I know it's suppoused to be very hard, but at this rate I won't be able to do anything at all.
Hope it helps/makes any sense