What are "Edosecha"?
Rashi: It is what Nevi'im testified and promised for Your Bayis, which is "Na'avah Kodesh."
Radak: They are words that Your Nevi'im wrote that testify about Your kingship.
Malbim: What You Yourself testified.
What is the meaning of "Ne'emnu Me'od l'Veisecha"?
Rashi: [What Nevi'im testified and promised] for Your Bayis is faithful. He looks forward to them, even though the days [of Galus] have been very long; they (the promises) are very faithful to Hashem.
Radak: They will be fulfilled very much then; no one will have a doubt in his heart about them.
Malbim: What You Yourself testified is more faithful "than the voice of great mighty waters" (verse 4). The water testifies that You rule over Ma'areches ha'Shamayim and nature, and conduct them like Your will, and You are mighty above. You testified that not only are You mighty above, like one who conducts from afar. Rather, "l'Veisecha Na'avah Kodesh"; You dwell in the Beis ha'Mikdash and conduct wondrous Hashgachah above nature. This is called Kodesh, which is separation from nature. This conduct is in Veisecha - via dwelling in the Beis ha'Mikdash.
What is "Na'avah"?
Rashi: It is like "Ne'os Elokim" (83:13); [also] it is like Na'aveh. It is not an expression of Naveh (beauty), for that never has an Aleph.
Radak #1: It is an expression of beauty. Here, there is Chataf Patach under the Aleph; elsewhere, it is silent, e.g. "Navah" (33:1, 147:1), "Navu" (Shir ha'Shirim 1:10).
Radak #2: It is an expression of desire. Everyone will desire Your house, which is Kodesh for length of days - "Nachon Yihyeh Har Beis Hashem b'Rosh he'Harim v'Nisa Mige'va'os v'Naharu Elav Kol ha'Goyim", "Yavo Chol Basar Lehishtachavos Lefanai Amar Hashem" (Yeshayah 2:2, 66:23).
Why does it say "Hashem l'Orech Yamim"?
Radak: It will not be destroyed again. 'Hashem" calls to Hashem.
Malbim: You do not do miracles [in Your house] occasionally, rather, constantly.