okay, sounds like your having a few problems!
Firstly that caliper, your going to need to try two things, firstly see if you can get a 'easy out' which is basically a reverse threaded drill bit, this may get the snapped end out. You'll need to get some penetrating oil and heat on it first though. likely hood is this WONT work so dont worry!
The standard fix is to simply drill it out, tap a new hole and put in a bigger bleed nipple, they are available in literally HUNDREDS of sizes so go speak to your car spares place to get some different nipples and then you know what sizes to drill and tap to. I think you need both a course and fine tap to get a good seal!
When you say the rear calipers are weeping, where from? I cant say i've ever had problems with the calipers but then again i've never bothered to bleed a BX and never had any problems
they should really be done as i suspect the rears will be full of crap, but the brakes work rather well so im not gonna go fixing it until its broke!
The drive socket, well dont give up too soon! Best thing you can do is deem the hub as a total loss and get to cutting, but i'd give it some serious heat and abuse before going that far! You dont want to go killing a BX16v drive shaft as they are bigger and difficult to get hold of! THe hubs are the same as are the calipers for all bx models (for the front calipers its only the 'hoop' that is different), so if you never get the bolt off its not the end of the world!
Have you tried a cold chisel? reasonably pointy, hit it into the nut face at 90degress to get a dint, then hit it fromright to left at an angle to 'undo' the bolt, you can get some serious force in like this but becareful when you slip cos it hurts a hell of a lot! The second it moves, even only by the smallest amount then you get the breaker bar back on. If the socket is an impact socket (black) then beat it back onto the nut and try undoing, if you have a 'normal' socket then you will likely have to make the sides of the nut flat again with either a grinder or a file!
If you do decide to cut it then cut into the nut parrallel to one of the sides about half way accross and half way through. Then put the chisel into the gap you've cut in. At this point there should be no damage to anything but the nut. Beat the crap out of the nut using the cold chisel and the big hammer and get the nut to partly snap off! Then try undioing the nut by hitting from right to left with the cold chisel again. Normally this gets enough heat into the rust and enough force to break the corrosion and get the thing moving!
As for the accumulator what are you trying to do? normally the accumulator is NOT on hard. You are trying to unscrew just the green ball from the pressure regulator and not tryiing to 'undo' the top of the PR right? If it really wont budge then charge the system upto pressure and try to get it to crack, you only want it to move a fraction of a turn. If you get desperate resort to the cold chisel and beat it in an anti clockwise motion. Once it's cracked then depressurise the system and ge tthe chain on it!
Once she/he starts lifting everything should bleed OK.