Get a good night's sleep, and don't worry. I doubt very much you will need a new accumulator. Bleeder valves are common stock parts..just need to make sure you get a replacement bleeder valve with the right threads. The trick is going to be getting the part that sheared off out of your accumulator; I've never had to do that, so hopefully someone else who HAS can chime in. It's probably going to make a pretty good mess because you now have no way of really directing or controlling the flow of the fluid as it escapes, but messes can be cleaned up.
Possible courses of action (and I'm just guessing here... don't do any of these until someone who's a better mechanic than I validates these guesses..)...
1. Carefully drill a small hole in the center of the valve still stuck in your accumulator.. use a cobalt drill bit, maybe 1/8". Go slow. Thread an "EZOut" into it and try and back it out. LOTS of PBBlaster. Let hours, or even days, go by.. let that puppy soak. Less muscle, more patience. If it does loosen and starts to come out, do NOT be in a position to get sprayed by the fluid. It's going to come screaming out.
2. If no joy, increase the bit size. Choose a bit that is almost as big as the body of the valve would be if there were no threads... in other words, you're trying to drill out the solid portion of the valve, but leave the threads intact, mated to the receiving threads of the accumulator. Once the solid part is drilled out, you'll experience the screaming fluid escape. Again, take care. Then you may be able to use a dental pick to extract the remaining threads. If you booger up the threads in the accumulator, you'll have to rethread them with a tap.
OR...
If you haven't leaked any fluids yet, your system still works, and your car is driveable, take it to any reputable garage. Those guys will have had to fight battles like this many times, and will know how to get it out and get a new bleeder valve installed. They might grumble and act all superior and stuff, but that's OK. Maybe have them check the other bleeders too to make sure they're not seized.
A hint when dealing with seized, rusty stuff... in another thread, someone mentioned using constant, steadily applied and increasing pressure. That person has obviously never lived anywhere near the rust belt, USA; that's terrible advice, and a sure fire way to end up with your current problem. What you want to do is break the bond the rust has. You're on the right track w/ PBBlaster... soak it, tap it lightly with a ballpene hammer (or anything metal, really), then walk away. Do that several times over a few hours. Next, and VERY important.. make SURE you have the correct size wrench, and that it's FULLY seated flush over the nut/bolt/bleeder/whatever you're trying to turn. If that area is rusty, scrape the rust off. Get the bolt/nut/bleeder looking as clean and hexagonal as humanly possible, and get the wrench SEATED without any play. If the bolt/nut is BADLY rusted and misshapen, you may be able to get the next size smaller wrench on it. If it's a metric bolt and your metrics are either too lose or won't go over, try an SAE. GET IT ON with the best fit possible. Now, when you're ready to give it a try, jerk it suddenly, like you're trying to sort of karate-chop it off, not lift a heavy object. You could try tapping the handle of the wrench or ratchet with your ballpene hammer.. not whacking it really hard sledge-style, but just a solid tap. Repeat. THIS IS WHY impact wrenches work so well, and why you're less likely to break things taking them off w/ an impact driver than you are with a ton of leverage. If it doesn't come out after a few taps or you find the wrench starting to round off the bolt, step away, soak some more, and try again later.
If all that doesn't work, then you're looking at heating things. That's a whole other discussion, and I'll defer to those more knowledgeable there, too.
But no, in terms of your accumulator, all is not lost. Just not as easy as originally hoped.
