Was the Ovalized tube just a styling thing to set it apart from others? - In theory, they were (and still are) used to maximize torsional stiffness. You'll find that several builders still use the ovalized boom tubes -- but do so with a larger diameter tube -- and you'll still find examples of the aforementioned tandems with either ovalized top and bottom or in the case of Lippy, an ovalized internal (aka, direct lateral) tube. The difference they make in how stiff and/or comfortable a frame is at best subtle and appreciated by owners of those tandems and at worst of no real consequence / difference than a round tube.

The fork spacing is wider on this, and the front hub is threaded. Fork spacing could either be a function of simply using a wider touring crown for added strength, or it could have had something to do with the threaded front hub which "may" have been used to allow for the use of a Phil Wood Disc Brake, which had a short lived run from the late 70's to early 80's: it was not Phil's finest product so be glad that it's not on there. That said, the Magura HS66 hydraulic rim brakes w/aero brake levers that ended up on the bike will have more stopping power than just about any other rim brake you'll find. Cannondale actually fitted those as OEM brakes for a few years in the late 90's.

The pinch bolt is on the front of the captain's seat post That's not unusual for most tandems and I've never really asked why it's done that way. Intuitively, I suspect that if the slots & key hole were added after the frame was made it would certainly make it easier to cut-in on an obtuse angle vs. an accute angle. Then again, a lot of frame builders add the slot & key hole when they're mitering the seat tube(s) and before the frame is fully assembled, in which case it wouldnt' matter. So, at this point, I suspect it's just how it's done on tandems.