I would throw my hat into the ring for a big oil leak, as in my experience, none of the other internal engine problems would not consume that much oil (~100 miles/qt?) without really causing the engine to run very poorly or showing mucho oil in the coolant. My 91S, when I bought it, consumed oil like this but turned out to be a big leak from the back side of the engine. Previous owner hadn't bothered to fix it, but I knew it would be easy. I suspect the oil filter is not tight enough, should be firm hand tight with gasket lubed with oil. Do not use a wrench to tighten. Or maybe it is a defective filter.
One thing to check for is whether or not you had removed the old oil filter gasket (rubber ring, usually stays with the old filter) before you installed the new filter. Friend had a Milano where it ended up with a big oil leak from the filter area. Turns out the old filter left it's gasket behind, and so he had double gaskets, which ended up shifting after a while and letting a lot of oil leak out.
Hot idle oil pressure, with 15W-50, in the 94-95 engines should read somewhere from 15-25 psi if you have a properly working sender. If the sender has faded, the pressure could read 0 to 10. What is the pressure at cruise, ~3000 rpm. Should be about 45-55 or in that range. Otherwise, with a bum sender it might read as low as 20-25. Evidently the oil level sensor in your engine must not be working, as it would show a warning light if you had run the engine that far down in oil.
Not to worry if the temp at idle rises to about 225F. That's ok (my 94LS does the same thing but everything is working fine) but you might check to see if the radiator fan comes on when it should. These V-6's run hot (190-225F) as normal. Colder is not best for the engine.
89 Milano (wife's daily driver since 1989, Shankle Sport)
91 164S (my daily driver since 1994)
94 164LS (~Q) (trip Alfa since 2000)
72 Morgan 27 (water time since 1976)
previously owned since 1964:
62 Morris MiniMinor 850, 67 Austin 1275 Cooper S (Downton 3/4 race), 64 Giulia Sprint GT (1st red one made), 72 Fiat 128 Sedan, 75 Alfetta Sedan, 78 Alfetta Sedan, 78 GTV, 81 GTV6, 86 GTV6