Can confirm. I find random ink sacs on river shores often.
Can confirm. I repeatedly get eff. 1 or quick charge 1 on books.
Can confirm. I have had 3 villagers who were heavily protected (by 50+ iron golems and torches in a wide radius) and were nametagged and traded with often who just randomly disappeared.
That is an intended feature.
Can confirm for pre-5 as well. Tested about y = 120 over the ocean, no caves could possibly be nearby.
It doesn't seem to happen anymore.