The mortgage tax deduction and the Homes Sales Tax exclusion should be eliminated. They are regressive pieces of legislation that serve to widen inequality.
The capital gains exclusion I have less strong feelings about, but it amounts in large part to excluding being taxed on the effect of inflation.
Landlords can deduct mortgage interest just like any profit-seeking business can deduct interest on loans taken on in furtherance of their business. That’s true for single-family homes or duplexes in a suburb same as a rowhouse, 4-plex or 50-unit building in a city.
The owner-occupant deduction applies to their unit, whether that’s all of an SFR, half a duplex, one level of a triple-decker, or 1 unit in a 4-plex.