The Gemara here mentions, regarding entering a Bayis Musgar in the normal fashion, "Derech Bi'ah Asrah Torah."
Is it really prohibited to enter a Bayis Musgar? Where is the Isur written? It is a Chidush to say that we learn a Lav from the word "v'Hisgir."
However, I noticed that the Ritva (Shavu'os 17b) says that one is "Chayav" for entering the house in the normal fashion.
Pesach Feldman, Yerushalayim
(a) Regarding the wording "Asrah Torah" - the Gemara may not mean that it is literally prohibited to enter the house. The phrase is a borrowed term, the source of which is in a Beraisa cited later in the Sugya discussing the laws of entering the Mikdash when Tamei. The Ritva, who says "Chayav", is also referring to a person who enters the house and then enters the Mikdash while still Tamei.
(b) As for whether it is actually prohibited (an Isur Aseh) to enter the house based on "v'Hisgiro" - this would seem to be a Machlokes Rishonim. The Rishonim argue whether the word "Hesger" means literally "locking up" or "closing" the house (or the person) with the Nega, or whether it simply means to "remain in doubt" or to "scrutinize" the Nega. For starters, see the Mishneh la'Melech (Hilchos Tzara'as 14:5) and the Mishnah Acharonah (Nega'im 1:3), who cite many of the opinions in the Rishonim on this matter.
If the Torah commands the Kohen to indeed lock up the house, it is reasonable to suggest that the words of the Gemara should be taken literally and it is prohibited by the Torah to enter the house, as you mentioned.
Best regards,
Mordecai Kornfeld
Kollel Iyun Hadaf