1)
Who is fortunate?
1.
Rashi: One who will reach to dwell in Your Bayis, "and praise You in it."
2.
Radak: (a) The Kohanim who used to dwell in Your Bayis, when it stood. (b) The Chachamim and Chasidim who isolated themselves there.
3.
Malbim (4-5): [If birds, which do not speak, find happiness singing to Hashem in a house or nest (refer to 84:4:1:6), people who anticipate dwelling in Beis Hashem, all the more so they are happy!
2)
Why does it say "Od Yehalelucha"?
1.
Radak: The will forever praise You, their entire lives. The same applies to "Od Kol Yemei ha'Aretz" (Bereishis 8:22).
2.
Malbim: Birds merely tweet, but people praise You, to speak of Your praises and winders!
