- "[Verse 46] indicates that after the infection has become known, all who have been inside the house are unclean, and instructions are given for their cleansing."
Actually verse 36 explicitly states that everything in the house is still clean before the priest declares the house unclean, which is why everything in the house is to be removed before the priest enters:
33 And the Lord spoke to Moses and Aaron, saying: 34 “When you have come into the land of Canaan, which I give you as a possession, and I put the leprous plague[a] in a house in the land of your possession, 35 and he who owns the house comes and tells the priest, saying, ‘It seems to me that there is some plague in the house,’ 36 then the priest shall command that they empty the house, before the priest goes into it to examine the plague, that all that is in the house may not be made unclean; and afterward the priest shall go in to examine the house. -Leviticus 14:33-36 (NKJV)
- "It seems that declaring a house unclean does not makes it unclean, but rather it's the presence of leprosy in the 'stones' that makes it unclean. At the point of discovering the infection, the house is already unclean."
The house is only unclean after the priest declares it unclean, which is why anyone who enters the house after it is quarantined by the priest becomes unclean:
37 And he shall examine the plague; and indeed if the plague is on the walls of the house with ingrained streaks, greenish or reddish, which appear to be deep in the wall, 38 then the priest shall go out of the house, to the door of the house, and shut up the house seven days. [...] 46 Moreover he who goes into the house at all while it is shut up shall be unclean until evening. 47 And he who lies down in the house shall wash his clothes, and he who eats in the house shall wash his clothes. -Leviticus 14:37-38, 46-47
- "But the priest who entered the house and identified the leprosy seems to have been left out from the 'cleansing' ritual. Why? Was the priest not made unclean?"
It doesn't matter if the priest was made unclean or not because he would wash at the Bronze Laver as soon as he got back to the Tabernacle:
17 Then the Lord spoke to Moses, saying: 18 “You shall also make a laver of bronze, with its base also of bronze, for washing. You shall put it between the tabernacle of meeting and the altar. And you shall put water in it, 19 for Aaron and his sons shall wash their hands and their feet in water from it. 20 When they go into the tabernacle of meeting, or when they come near the altar to minister, to burn an offering made by fire to the Lord, they shall wash with water, lest they die. 21 So they shall wash their hands and their feet, lest they die. And it shall be a statute forever to them—to him and his descendants throughout their generations.” -Exodus 30:17-21 (NKJV)
But to answer your question, the priest is not made unclean because he does not enter the house again until the seventh day:
39 And the priest shall come again on the seventh day and look; and indeed if the plague has spread on the walls of the house, 40 then the priest shall command that they take away the stones in which is the plague, and they shall cast them into an unclean place outside the city. -Leviticus 14:39-40 (NKJV)
Only those who enter the house while it is shut up during the seven day quarantine period become unclean.