If you gently roast it, the bitterness goes away and the toasted "nut" tastes very much like a peanut. Here in the San Francisco Bay area we have Bay Trees, the source of the bay leaf used in...
Yes, they are safe to eat. Just like when an apple slice has been left out turns brown, avocado exposed to air turns brown as well. Try to consume your avocado the same day you cut it.