just because a bow is not center shot,, does not always indicate need for light spine arrows,, if you have to shoot a 35 spine arrow from your 50# bow then something is really not matched( or you may not be drawing the bow close to 28),, maybe the bow is not really 50# or has a cast similar to a 35 pound bow,,
a well made 50# bow,,not center shot will shoot at a 29 inch arrow 50# or 60# spine drawn to 28 inches very well, I would have to say that going down in spine is not necessary most the time,,, it may be possible that you are not drawing the bow to the draw length that would make the bow weigh 50#,, making the spine of the arrows too stiff and shooting to the right
and yes the string alignment is probably part of the problem,,I have been making bows for more than 20 years and have never had to shoot a 35# spine out of a 50# bow to get perfect arrow flight,,