Castings may become toxic if they are allowed to go anerobic by cutting off the oxygen to them by letting them get to wet and compacted. castings by themselves offer no food source for the bacteria to process after they have been processed several times. As long as they are aerated and continue to have organic material mixed in with them everything should be fine.

So long as you add more bedding and food into it you should be ok. The main issue is that no creature survives well in an environment composed entirely of it's own waste. If you wish to harvest a bin of pure worm castings with almost no bedding material or worms then you might get that by leaving your worms in just their own casting with no way to escape to a better environment. Harvesting pure castings often means the worms don't survive.

I like to do my bin by halves, that way the worms can move to the new side as the more mature side starts getting too full of finished castings. My bin is 4' long though. I'll feed mostly in one half of the bin until things start looking like mostly castings. The I start feeding in the other half. The hope here is that the worms will leave the mature casting half of the bin and make harvest easier. I have harvested this way once. I still had to pick lots of worms and cocons out of the castings but it sort of worked.

Here is a link that might be useful: My Bin