Why is it necessary to clear the Kelim from the house before the Kohen arrives?
Rashi #1: To prevent them from becoming Tamei, because until the Kohen arrives and declares the house Tamei, they are Tahor. 1
Rashi #2: Refer to 14:34:3:1*.
Seforno: It is to allow the owner time to do Teshuvah and for him and the Kohen to pray to Hashem for mercy. 2
Seforno: Bearing in mind the immense value of a house as compared to that of a garment. A Midrash says that the sequence of events regarding a stricken house hints at the destruction of the first Beis-ha'Mikdash, its rectification via the building of Bayis Sheni , the destruction of Bayis Sheni and its purification via the building of the third Beis-ha'Mikdash - may it come about speedily in our days (refer to 14:34:2:3).
Why must he remove the Kelim, lest they become Tamei? Why can the owner not simply immerse the Keilim after they become Tamei?
Rashi: The Torah has pity on the earthenware vessels, for which Tevilah does not help. 1
Rashi (Yoma, 39a): If the Torah has pity on small things, how much more so on big things; if it has pity on the money of the miserly, how much more does it have pity on the money of Tzadikim! Moshav Zekenim - also Tamei Kli Cheres are useful for when he is Tamei! (Perhaps there was a glut of Tamei Klei Cheres; there was no value in having another such Kli. - PF)
Why must he remove the Kelim, before the Kohen arrives? The Kohen can see it, and be silent until the remove the Kelim. Nothing is Tamei until the Kohen leaves the house and declares it Tamei!
Moshav Zekenim citing R. Yehudah ha'Chasid: The Kohen would not be concerned [to wait until they remove] an Oni's Kelim, only for an Ashir's Kelim, and this is brazen. Therefore, the Torah equated them. 1
The Torah could have equated them, and always said that he may not be Metamei until after they remove the Kelim! Perhaps it would appear ludicrous, that the Kohen hints that they remove Kelim because it is a proper Nega, and the house is not Tamei yet. (PF)