I'm sorry I just don't have the expertise to answer most of your questions. All I can say is that the wheels noticeably lean inwards, and this seems to be getting progressively worse.
If it's that visible, it's pretty bad and I don't think a camber gauge will be any use!
I doubt the bushes would cause that. If the bushes were totally shot, the axle would still maintain the camber angles when static, but the whole axle would slop about, causing incorrect angles on cornering and probably accompanied by some horrible knocking and groaning.
One thing's for sure - the corrosion underneath generally is pretty bad.
If there's a lot of corrosion, one problem with beam axles is that they can corrode, weaken, and can even snap or welds give way. I'm sure another member has had this problem with an old Jazz, not sure if they scrapped the car because of it. If it was really bad, it could certainly deform and cause the rear wheels to lean in, but I'd be surprised for it not to be picked up by a previous MOT if it was that bad.
Any chance of some photos underneath, particularly of the rear beam, which is a metal assembly that sits across the car, just in front of the rear wheels? If only to rule it out - if the axle is collapsing, the car really isn't safe to drive.
From what you have said, I'm pretty sure it's not bushes, but aftermarket ones are about £20 each, two required, plus new bolts, probably another £10-£15. The problem is labour, it's a big job, especially if the garage wants to take the axle off completely to do it. You could soon run up a fairly hefty bill.
If the axle is rotten, then the only option is a good secondhand one as suggested above, probably around £75. Again, it's a pretty significant job, and a garage would probably want half a day to do it, at £40+ per hour.
As Culzean says, it could also have been seriously overloaded at some point in its life, the rear arms are not that strong and both could conceivably be bent by such an event, but I don't think you would see it deteriorating afterwards.
Somebody really needs to poke their head underneath and have a look what's going on.