It's offset because that is where there was something to hook to on the chaff spreader.  The piece which goes across where the motor bolts on was not strong enough, in my estimation, to handle having the hitch load.   Having it offset is not a big deal, but it does make it so that you can see the header behind you a little easier.  It also gives you a little more room for the snouts which are sticking out on the other side, and like you said, leaves the trailer on the road a little longer when pulling over.   Those were side effects of just making something that will work, though. 

Tabs are welded on.   I'm not inclined to get all worked up about that type of thing.  If the weld is not botched I don't see it causing a problem to begin with, plus the axle is 1/2" thick and the tabs are not welded to a critical portion of the beam.  I could have welded the tubing straight to the axle, but that would have been an overhead weld and would have been welding to the critical portion of the beam.  It's been on there for a number of years now.