To my mind, the only thing wrong with this plan is that you are confusing your terms --- that actually matters, practically.
Broth and stock are not fundamentally different things. When French-style chefs get technical about a distinction, what they usually mean is that stock has a certain significant amount of gelatin in it (naturally extracted, as a rule), and is thus sort of halfway gelled when cold, whereas broth is usually not like this. Sometimes the distinction is more that "broth" is pretty much anything goes, and stock is more specific.
The distinction you're making is between a white stock and a brown stock, and that's quite a different matter. Some points:
1. Brown stock with chicken alone does not work well. It ends up having far too much distinctively chicken flavor. That sounds like a good thing, but it's not: a brown stock should be useful as a backdrop for a wide range of sauces, not a flavor of its own. Chicken (or turkey) can be used to extend meat (esp. veal and pork), but should not dominate. White stock with chicken alone works fine, though purists would disagree.
2. A pressure cooker works admirably for making stock, and you can keep it going for hours if you wish. The crucial point is that the longer you're going to run it, the more scrupulous you need to be about removing all the fat before you start. Bear in mind that a pressure cooker on max pressure will tear the collagen from the bones and give you a very stiff stock, so if you overdo it on bones you could end up with something very stiff but not especially flavorful. The ideal is balance.
3. Excellent stock should include three basic ingredient categories: aromatic vegetables, meat, gelatin/collagen. It is usual to get the gelatin from the collagen in bones, but it can be added with pure gelatin if you prefer. What I notice in your post, which I've seen again and again -- it seems to be something perpetuated by third-rate cooking instructors -- is the wrongheaded notion that stock requires bone and not meat. This is utterly wrong. You can make a pure bone stock, but it will never be as flavorful as a meat stock. Cheaper, though, which is the usual point. The best chicken stock possible uses an entire chicken, minus the feathers, lungs, skin, fat pads, and liver -- these either cloud the stock (fats) or make them bitter (liver, lungs, etc.). Head, neck, heart, tongue, feet, wings, meat, bones --- that's all good. But that's an expensive way to make stock, so what you usually do is put all the trimmings you're not using for your chicken dish aside to make stock. In the end, the meat flavor comes from meat, not bone. The gelatin comes from the collagen in bones, and to a lesser degree meat. The background roundness, depth, and complexity comes from the vegetables.
4. Vegetables: I think 1/2 weight of vegetables to meat/bone works well, but it does depend a good deal on which vegetables. Some vegetables should be avoided altogether: all leafy greens will make your stock taste of "pot liquor," which is delightful but excessively distinctive to the point that you can't use it for anything but greens. Most squashes, certainly hard squashes, will overpower a stock in an instant. Peppers, sweet or hot, tend to make stock bitter. With some vegetables, it's a matter of quantity and balance: turnip, radish, daikon, and other members of that family can, if used in too great a quantity, overpower your stock, which will then taste forever of boiled turnip. Potatoes, in significant quantity, will break down into a cloudy fog of tiny particles that will not strain out well --- and when they do, they will take a lot of your good stock with them. Carrot is good, but in large quantities will make your stock an intense golden color that is not always desirable. It's probably impossible to overdo celery or leek. Incidentally, on the subject of spices, a bouquet garni is good, but don't overdo it. Some whole peppercorns are good. Strong spices, like bay leaves, I tend to think overpower the stock. Salt is very bad: you want to be able to manipulate your stock's characteristics when cooking, and if there's salt in it you cannot do this.
5. Bone-roasting: If you want to make brown stock, you need meat bones. Best is veal, then pork, then beef. Roast them for an hour at 450, in a tray that just holds them. Turn them and roast another hour. Turn, scatter with half the vegetables, and roast another half hour to hour, depending on how the vegetables look. Deep brown is good; burnt is bad. Your bones will look so deeply browned that they verge on burnt, but they should not actually burn like this. Put them in the pot, then deglaze the pan with water and pour all the deglazing liquid into the pot. Add the remaining ingredients, add cold water, and cook as stock. Note that this doesn't work so well in a pressure cooker because it's only worth doing with a lot of bones --- but a pressure cooker can't actually hold all that much at one go.
6. Cooking: Cook slooooow. You do not want the liquid to roil, you just want a bare simmer, the odd bubbling around the edge. You should have to look carefully to see whether it's simmering. Fish: 1 hour or so. Chicken: 2 hours or so. Meat: 4 hours or so. Brown stock meat and bone: 10 hours+. You can increase these times, but fish stock and to a lesser degree chicken stock will lose their bright, fresh flavors if you cook them too long.
7. Straining: Strain coarse, pouring fast, to "wash the bones" of any good flavor. Strain again fine, removing all the little traces of broken-down bones and vegetables. Now you need to chill this liquid fast, or it will act as a brilliant medium for bacterial cultures. The fastest method for normal use is to put the pot into a big sink with the plug in, pour a whole bunch of ice around the outside, and then add cold water to the sink until the pot almost barely wants to float. Wait half an hour and test the temperature with a scrupulously clean finger. If need be, drain the sink and add more cold water (and ice, if you've got it). When the liquid is about room temperature, put the whole pot in the refrigerator overnight. DO NOT STIR. In the morning, the fat will have frozen solid on the top and can be scraped or strained off. Now you have stock, cold and gelatinous, and can decide what to do with it.
So what about your plan?
Keep it simple. Start by stripping off the fat and chopping your leg quarters into coarse chunks. Put those in the pressure cooker. Add half the weight in vegetables, heavy on the celery and leeks (and other onions), some carrot, light on other things. Add a few peppercorns and a bouquet garni. Fill the cooker 60% full with COLD water (important: do NOT ever put hot water in!). Seal the cooker, put on maximum pressure, and crank it up. Process normally for 2-3 hours, then natural slow-release (shut off the heat and wait until the button goes down). Strain the liquid coarsely, then fine. Chill in the sink, then in the fridge, then scrape off the frozen fat.
Now bring your stock back to a boil, noticing how it clears as it warms. Skim off any scum that rises when you do this.
Taste your stock. It should have a paradoxically minimal flavor. It should, that is, have a distinct taste --- which you can't quite place. It's definitely meaty, but somehow it doesn't really taste like much of anything. That's perfect. When you use this in a sauce, you'll add stuff, and that stuff will be the core of the flavor; the stock will be the backdrop that brings it to perfection. When you use it in a soup, the same happens. And so on. When cold, the stock should be definitely gelatinous, but not rubbery --- not quite Jell-O, but not just thick liquid either. Now if your stock has too little flavor or too little gelatin, you can reduce it by boiling rapidly down by a third and testing again. If it's too thick or too strong, add water.
When it's right, simply can it under pressure, the normal way. If in doubt, you can go overboard with processing time: there is nothing to break down, so you can't overcook it. Remember just how good a bacterial medium stock is, and don't take chances.
If you want to use the pressure cooker to process stock but have too much material at one go, there is an alternative, very excellent approach. Simply do this whole process the normal way, using only enough material and water to fill the cooker to its safe 60% level. Strain, cool, chill, and scrape as usual. Now do it all again, but instead of using cold water, use the stock you just made. Do this as many times as you like. The stock will get very, very strong as you do this. In the end, you may have a stock that actually does need to be diluted. But you may have something else.
If, in particular, you have done this and used browned meat bones, look at the results after 3 or 4 successive processing runs. It should be deep brown, and when warmed it should be crystal-clear. Cold, it should be very thick, stiffer than Jell-O. It should have an extremely intense meat flavor, with that same strange not-quite-identifiable thing going on, and yet by now it should have a real flavor of its own that is mysteriously meaty. Warm, it should be thick, like medium gravy. If you have achieved this result, you now have a true demi-glace. Can it as it is, in small cans: you never need a whole lot of this at one time, so don't put it up in quarts. Half-pints would be ideal.