It sounds like you were expecting slow cooking to be like sous vide. Well, it's not. The point isn't controlled sub-boiling temperatures, it's something on the border between simmering and boiling for foods that just need a long time to cook at that approximate temperature.
Slow cooker recipes are not supposed to be very sensitive. They're expecting to be approximately boiling for most of the time, and the difference between low and high is pretty much whether the boil is marginal or a bit more substantial. In many cases, this just matters because the quantity in the pot varies, and it takes more to keep the stuff at the top hotter if it's farther from the bottom, or if it hasn't all cooked down into the liquid yet. In either case, it shouldn't be a full rolling boil; it's just boiling on the bottom, so the rest of the liquid is probably a bit below the boiling point. And yes, this is still slow cooking. It's not boiling fast enough to lose a huge amount of liquid (or worse, boil over) with the lid on.
I wouldn't really try to assign temperatures to slow cooker recipes. Like I said, they shouldn't be that sensitive. If your bean soup recipe didn't work, maybe it was a bad one. But "reliable" in the context of slow cooker recipes doesn't mean "exact times and temperatures". They're generally things that will be perfectly fine if you cook them 25% longer. Not everything in the kitchen has to be precise and formulaic; slow cookers and slow cooker recipes take advantage of that fact. And even if you do try to calibrate, you'll have trouble, since there's a temperature gradient from bottom to top. Unlike sous vide, a slow cooker is not constantly well-mixed. In equilibrium it'll be boiling at the bottom, and 10-20 degrees cooler at the top.
If you really wanted to use a sous vide controller, I imagine something like 95-98C would work for basically every slow cooker recipe, no matter whether they say high or low. Of course, mixing thoroughly enough to make your controller actually work, you may be overstirring whatever you're cooking. But the point is, things you cook in a slow cooker aren't really going to care much what the exact temperature is; it just matters that it's hot, near boiling, and not boiling so fast that it sticks on the bottom or loses a lot of liquid.
Finding reliable recipes... Well, it's like anything else. If you're looking on the internet, you have to learn to judge for yourself and look for warning signs, or stick to sites with lots of reviews. You also have to accept that sometimes you have to test for doneness and be flexible about time. This isn't really unusual; baking recipes should always have some kind of test ("until golden brown") and the actual baking times will vary. (With something like bean soup, sure, maybe the recipe was bad, maybe you didn't soak enough, maybe the beans were a little different. A stovetop recipe wouldn't have been precise either.) If all that isn't good enough for you, buy a slow cooker cookbook; tons of those have been published in recent years.
Braises are, by their very natures, cooked to well done. They achieve an internal temperature well above 165 F which will make every meat well done.
Please see:
Why would a pressure cooker shorten a braise time?
for a detailed discussion on how the collagen to gelatin conversion takes place over time, and is made faster in pressure cookers.
The fact that the cuts that are suitable for braising are exactly the ones that allow the slow conversion of collagen to gelatin is what makes the pressure cooker suitable.
The real issue with cooking braises low and slow in the conventional oven is that at sea level, the internal temperature cannot get very high, so you need time for the gelatin to collagen conversion to take place. There isn't much point in raising the oven temperature higher than required to allow the time for this conversion, and it prevents overcooking or drying out the outside of the food while the conversion takes place. The meat will still be quite well done; it is only moist and succulent because of the melted fat, and the gelatin lubricating the meat fibers providing that slow cooked unctuousness.
In a pressure cooker, the ceiling temperature is raised, so the time can be shortened. The inside of the food is still quite well done. No loss to quality (since the food would be well done anyway), but a much shorter time.
If the food was not suitable to be cooked well done, the pressure cooker would be far from ideal, as it would certainly overcook the inside of the meat.
Note also that you have listed two special cases where foods are cooked at lower temperatures to avoid agitation (from the bubbling and boiling of the water), rather than because of issues directly related to time and temperature:
Stock. Stock is cooked at a simmer to avoid the turbulence and circulation from the bubbling and boiling leading to more dissolved, emulsified or suspended particles in the final product. That is, the goal is to keep the stock clear instead of cloudy. If you are not concerned with this aspect, it can be cooked at a full boil.
Beans are cooked slowly for several reasons, of which the main one is convenience. Cooking them at a full boil would require more attention (so they don't burn on the bottom), and is not terribly feasible in an oven, which is the easiest way to do them. Also, by cooking below the boil, there is less agitation in the pot, and so less splitting and sloughing of the bean skins, which some people find less than pleasing.
In the pressure cooker, you will not get this kind of agitation, because once the pressure is achieved and the food is at equilibrium, it is not going to be going at a full boil, but more of a simmer, but a much higher temperature simmer than is possible at sea level pressure.
Best Answer
There are several different categories of these devices, but they do all tend to look quite similar. Prices can vary by a very significant amount, though. So.
Rice Cooker
The simplest kind of rice cooker is the kind with just a switch on the front to turn it on. Mine is like this - when plugged in, it's in "warm" mode, press the switch and it goes to "cook" mode. Rice cookers are designed to boil their contents, then wait until the water's all absorbed/evaporated and switch back to a keep-warm mode. That's all you need in order to cook rice, so it's all a basic rice cooker will do. You can cook other things with it, if you bear this in mind.
Rice Cooker with other functions
Depends what the other functions are. I've most commonly seen rice cookers which can also be slow cookers, in which case they heat more gently and disable the automatic shutoff used when cooking rice.
Slow Cooker
A slow cooker, commonly also called a crockpot after a popular brand, is a device that looks quite a lot like a rice cooker but usually has a fairly chunky ceramic cooking pot instead of a thin metal one. Slow cookers are designed to heat up to a moderate temperature and stay there for long periods of time, allowing you to gently cook something like a stew or a shoulder of pork for hours upon hours without having to use the oven or dance attendance on a saucepan. Slow cookers often have two or more temperature settings - mine has "high", "low" and "warm", but they can come with more. A common extra feature is an even lower temperature setting designed for making yoghurt.
As mentioned above, a common combination is a rice cooker which can also be a slow cooker.
Pressure Cooker
A pressure cooker is a very different beast. Pressure cookers have a sealed chamber (typically with a big, heavy lid which you put on and twist to lock it into position) and a pressure valve. They heat up and form a high pressure environment, which raises the boiling point of water. Because the water stays liquid at higher temperatures inside the pressure cooker, food cooks a lot faster. A lot faster. This also has impacts on flavour and texture of food, sometimes positive, sometimes negative.
To answer one specific point in your question, you can cook rice in all pressure cookers, in much the same way as you can cook rice in all saucepans and all microwaves, regardless of if they have a "rice" button.
You can get stovetop pressure cookers, which require careful control of the heat from the burner to maintain the right pressure inside. People tend to be wary of these as we worry about them exploding, although modern cookers have excellent safety valves which will vent long before there's a risk to the integrity of the pot.
The easier kind to use is the electric pressure cooker, which heats itself up and takes care of maintaining pressure automatically. These come in a variety of models with more or less sophisticated features including multiple pressure levels, automatic venting/keep warm/cooling at the end of the cooking time, and various other cooking functions, which leads us to...
Multicooker
Any one of a number of devices which aims to combine one or more of the above categories, plus more, into one device. A rice cooker with a slow cooker mode is technically a multicooker. Some multicookers claim to do techniques usually done on the hob and be able to roast things and deep fry things. Whether this is successful or not is something you should really check the reviews about.
The popular multicookers these days are usually electric pressure cookers with more modes. They may have a hotter heat setting to use for frying and searing with the lid off, or a slow cooker mode which lets you put the lid on but won't build up pressure as it would in pressure cooking mode. Some have a rice cooking function, although typically they will pressure cook the rice rather than simply boil it as a standalone rice cooker does.
How to choose one
First, decide what you want.
Then, decide how much you can afford.
Then, read the feature lists. If a rice cooker can be used as a slow cooker it will tell you, because this is more expensive so they need to advertise that capability. Electric pressure cookers seldom come without some claims of being at least 6-in-1 devices. Mine's one of the most basic on the market and claims 6-in-1, in reality it's only capable of sauteing things (with the lid off) or pressure cooking them (with the lid on). So check carefully. Read reviews. There's no substitute for some research work in choosing a device for your kitchen.
You may wish to have multiple objects so you can use them all simultaneously - which means you can gradually acquire a very affordable slow cooker, rice cooker, pressure cooker, whatever, one by one. But that takes up lots of space as well, which maybe you don't have, so you might want to spend more money and get something which has many modes for different ways of cooking.