When it says "The Lishkas Metzorah is called that because metzoraim do tevillah there, and R Yehuda says not only them, but everyone," why can't this mean that this is the room where tevillah occurs THE EVENING BEFORE, and now, the next day, others can tovel there as well (You might have thought only metzoraim, ka mashma lan, everyone) It seems this explanation would deal with many of the kashas.
Also: Now when the rabbonan are talking about metzoraim needing tevillah, they mean, when they come for tevillah, not to the exclusion of others.
We asked a question on the explanation of the Chafetz Chaim, and you are suggesting an answer by saying that Rebbi Yehudah is only discussing the Tevilah of the evening and not of the morning.
This, however, will not suffice to resolve the Chafetz Chaim, because it is clear in the Gemara that the Rabanan who argue with Rebbi Yehudah do not require Tevilah for a normal person (other than a Metzora) at a time when Rebbi Yehudah does require Tevilah for a normal person. According to your suggestion, though, even Rebbi Yehudah should not require a tevilah for a normal person, since in the evening there is no requirement for Tevilah. Furthermore, the Rabanan must be referring to the Tevilah in the morning, since the only reason they hold that a Metzora is required to be Tovel is because he is "Dayish b'Tum'ah" and not because he has not yet been Tovel at all (which is the reason why he needs Tevilah in the evening).
In addition, the RITVA clearly states that the Rabanan maintain that a normal person does not need Tevilah at all. This is also clear from the Havah Amina of the Gemara which was discussing a Metzora "who did not immerse in the evening," and nonetheless the Rabanan do not require a person who is not a Metzora to be Tovel, even though he has not done Tevilah at all. (We do not find that the Gemara changes from this assumption; rather, the Gemara changes the assumption regarding a Metzora's requirement to be Tovel.)
Finally, according to the Chafetz Chaim who says that everyone needs Tevilah if he was not Tovel the evening before, why did the Rabanan say that the Mikvah was only for Metzora'im and not for all others (as is clear from the Mishnah cited in our Sugya) -- everyone else also needs to be Tovel if he was not Tovel in the evening! Thus, the word of the Chafetz Chaim require further elucidation.
I think I need more explanation: Isn't tovel for metzoraim d'oraysa, from pasukim?
Also, didn't we learn that everyone needs to tovel in the morning to remind himself of the day, etc...?
Therefore, what exactly is the machlokis? if the Lishkas Mitzoraim was in fact named for the place where mitzoraim toveled the night before, how would that affect the sugiah?
Yes, the Tevilah of a Metzora is certainly mid'Oraisa and nobody argues about it.
However, according to Rebbi Yehudah, it is not necessary for someone who was Tovel in the evening to go back and do another Tevilah in the morning because of a Gezeirah (of "Serach Tevilah"), since he was aleady was Tovel at night and that suffices to permit him entry to the Mikdash. However, the Sugya makes clear that Rebbi Yehudah does require a Metzora to be Tovel again in the morning. Because of this, the Gemara asks that Rebbi Yehudah contradicts himself, because earlier he said that it suffices if one was Tovel the evening before. (If, however, he was referring to the Tevilah of a Metzorah in the evening, the Gemara would not have asked anything for there would not have been any contradiction in the words of Rebbi Yehudah.)