~20cm Dolls Slow slither to sculpt successful joints, slightly spam but sincere!

I usually use wooden balls as bases for my joints. They are hard enough to withstand the tension for test stringing and eliminates the process of trying to freehand a perfect sphere. I try to find ones made from dense wood, because they are less likely to move with changes in humidity. You do need tools to cut the slits, though.
An easy way to shape the socket is to cover a ball with cling wrap or tin foil and just sculpt on top of it. The foil prevents the clay from sticking, leaving a nice socket once the ball is removed. As long as the clay doesn't shrink too badly, you should get a perfect fit every time.
A similar process can be used for torso joints or other places where you don't want to base the joint on a sphere. Just sculpt the convex part of the joint first, making sure to sand it and finish it to the desired shape, cover it and sculpt the concave part on top.

As long as the joint fits ok it shouldn't crumble, even with cheap air dry clay. Cracking and breaking mostly happens if a joint socket is too small or unevenly formed as the unevenness concentrates the pressure on the first point of contact. Breaking can also happen if you force a joint to go further than it is designed to go, for the same reason. As long as the pressure is evenly distributed over the joint surface, you'll be fine.

My rule of thumb fro predicting the change in joint behavior between a loosely strung original and a tightly strung cast dolls is this:
Increased tension amplifies the current behavior.
A joint that smoothly and reliably holds it's position when loosely strung will be even more reliable when tightly stung.
A joint that slips and snaps into it's extreme positions when loosely strung will be even more snappy when tightly strung.