So, generally, I’m going to say, rig first, as best you can, and just use moderate corrective shapekeys-- if you even need them!-- to adjust things. Let’s look at one of your shapekeys:
I don’t consider this moderate.
Now, let’s do what I said-- let’s apply some visual transforms on the targets of the drivers to see what the values are. I’m not going to bother figuring out your whole rig. Let’s look at the target of the driver, THIGH.L, apply visual transform, and see what we get:
Here’s one. Let’s adjust the IK target and do it again:
Are those numbers what you expected? You expected this to be rotating in two axes only.
Those numbers are discontinuous, even though the IK target angle is continuous. Your shapekeys are referencing X and Z of that bone’s rotation axes. But here, where the bone is getting rotated more than 90 degrees in one of its axes, those axes blend together. They’re no longer distinct. (Think about the Earth. We have 180 degrees of longitude but only 90 of latitude. -180 to 180 of latitude is redundant-- more than 90 degrees of latitude blends into longitude. It’s the same way with Euler angles. But Blender doesn’t “antialias” the same way we antialias latitude-longitude. It uses different techniques.)
And, if you trace these values through your drivers, you’ll find that both of your drivers are working, exactly like you told them to. That doesn’t mean they’re doing what you want though.
if I need a “butt bone”
Yeah, before you do corrective shapekeys, you want to be messing with bones. You’ve already been messing with bones quite a bit, enough that I’m not interested in figuring out what they’re all doing. I can see that you already have a deforming butt bone. But in general, yes, if you want more complex motion, you need to use more bones; and no, you can’t really replace them with corrective shapekeys. You need to make the bones do what you want. (And, yeah, you need to weight paint them correctly, but that’s usually a secondary concern, far easier than making the bones do what you want.)
My recommendations:
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Stop using corrective shapekeys. For now. Work on bones first. A well-rigged model does not need corrective shapekeys. Shapekeys are a crutch, which is reasonable, but it’s a crutch that only works when you already know how to do everything else. You don’t yet. So don’t try to use corrective shapekeys yet.
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Don’t test your model in unlikely poses. If you don’t want to make a model that can do a frog pose, don’t worry about frog pose. Know what you want to do, and build to do that; don’t build to do things you don’t want to do.
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If you do want to do a frog pose, then make a model and rig based around that. Make your model and rig such that you don’t have to rotate more than 90 degrees. Position your rest pose in the middle of likely deformations, so that your thigh, for instance, rotates ± 45 degees, not ± 120 degrees.
Re: weight painting: start with autoweights. Don’t get deep into Blender’s tools, they’re a trap. Use gradients at most to change weights from autoweights. Using more bones, properly oriented, is more important. But, it sounds like you’re having problems from unfamiliarity, which I can’t speak to without knowing what you’re doing, and you should consider asking focused questions regarding that. Your pic of your butt, weights don’t look great, but is it a reasonable orientation for the leg bone? See 2) above. If you pose your model poorly, it will look poor. Don’t pose your model poorly and expect your rig to take care of it. Build your rig to make good posing look good. It will never make bad posing look good.
