1. Regarding “Kever David”, it’s certainly a holy place, since so many thousands of Jews, over the course of many centuries, have sanctified it with their tehillim and tefillos and tears, but it is *not* David’s kever. It is *possible* that it’s over, or in the immediate vicinity of, an entrance to the tomb of the later kings of Beis Dovid, which was built somewhere in the western suburbs of Yerusholayim after the original royal tomb in Ir Dovid filled up. But it might be nothing but a place of Torah and Tefilah, with no graves at all.
2. Those who go to Uman for Rosh Hashono do so because Reb Nachman specifically asked his chassidim to do so, and promised that he would go to bat Above for anyone who comes. Therefore they cannot substitute some other place, whether in Eretz Yisroel or anywhere else. They are going to their rebbe, and he is not in some other place. It’s not for Rabbi Berger, whoever he is, to say that the same benefit can be had by going to “Kever Dovid”. Maybe it can and maybe it can’t, but there is no way he could know .