To visit Rav Kanievsky, I had to come to his shul to daven in the same minyan with him. Then I, along with other visitors, had to accompany him on his short walk from shul to his house. Then I had to wait for about half hour or so. At that time his wife (Rav Elyshiv’s daughter) was still alive; I saw her.
As for Rav David Abuchatzeira (whom I would like to visit him on my next trip to Israel), I was told that he lives in Nahariya and is available for visitors starting from 3pm during the week, except Sunday (corresponding to his last initial – A) and Wednesday (corresponding to his first initial – D). On other weekdays, he should be available.