1)
How did David know that Beis Hashem will be here?
1.
Malbim (21:28): (a) Hashem answered him; fire descended from Shamayim, and the plague stopped. (b) Hashem told him to offer there, even though the Mishkan and Mizbach ha'Olah were in Giv'on. David was forced to offer there (in the place he bought from Arnan), and there he was not afraid of the angel.
2)
Why does it say "v'Zeh Mizbe'ach l'Olah l'Yisrael"?
1.
'Rashi': From now and onwards, anyone who seeks Hashem to offer a Korban to Him, will offer it on this Mizbe'ach.
