Is it obligatory to refrain from entering a house built with mixed (halal and Riba) money, especially given a prior oath not to enter it if it were built with Riba? And what should one do to clear one's conscience of this money and fulfill the oath after giving charity as expiation for it?

Regarding the usurious loan: It is forbidden to take it, and one must repent from it. Nevertheless, it is permissible for the borrower to benefit from what was built with it, because the sin is attached to the borrower's liability, not to the money itself.

As for the repeated oaths not to live in the house built with a usurious loan, which have not yet been broken: If all of them occurred before the breaking of the oath, then, according to the preponderant opinion, only one expiation is required for all of them when the oath is broken. If the expiation has already been given - as mentioned in the question - then that is sufficient, according to the view of the majority of scholars who deem expiation before breaking the oath to be valid.