Nothing is new here, as far as I know.

UA site typically only brings up codeshare flights with * partners on revenue bookings. The only way I'm aware of to force what you are looking for is to search for an award, which always book on operator metal flight number only, and then during the process, you will get an offer to buy with $. Not necessarily on low fares (the partner may price into Y or other high bucket), but it should be there. Of course, if there is no award availability on the flights you want, then obviously, not an option.

As for booking the NH flight as a UA codeshare, if the only issue you have is the inability to select a seat online, forget about it. That part, AFAIK, has nothing to do with UA - I just don't think NH's site deals well with codeshares. It is not really "time consuming" to call and select a seat - the longest I've been on hold with NH is about 2 minutes, and they are always nice and happy to help with seat assignments. If you're beef with being on a codeshare is about something else, that's a different story. But over seat selection - that's seriously the least of your problems. This is much more of a problem with AC-operated codeshares, where the seats can't be booked online and hold times are 45+ minutes on a good day.

Orbitz and similar might work too - but note that if the first flight goes on NH metal, it will likely be booked on NH stock and therefore not eligible for PQD. So just make sure you are careful there if PQD are a concern. If they're not - have you looked at booking with NH's site - or even booking other * carriers - SQ, TG, etc.