Looks like you figured out the aiming, congrats.
Re- spine: What Bowmania said... it is way way too easy to have nock low. I have been fooled several times...
I was having very similar issue recently on a bow. So I experimented with spine stiffness of fletched arrows. I dont find that varying stiffness of fletched arrows moves POI significantly. I am sure it moves it a tiny bit.. and for guys shooting 70yds and shooting very consistently t likely matters. At 20yds barebow not so much for me. Various barebow guys have tested online and shown videos showing "not much difference" (Greg at 3D archery has one, others...) I ran 15 arrows, three each, 400-800 through a bow for a few ends to see what happened. Some with tiny vanes even.. just to see. I couldn't see any significant difference, with a 6" average group at 20 yds and the stiff and weak spines pretty randomly sorted in the bunch. I suppose if I ran experiment a 100 times, documented and tracked each spine impact point, etc I might find something. But I quit figuring that spine was going to move fletched arrow impact significantly given my own inherent accuracy, determined that issues I was seeing on my bow was ultimately a form problem and maybe a grip difference. So just started working on my consistency and form a bit with the new grip. Getting better now. I think I had my hand position wrong on the grip, way it is set up it doesn't like a low or medium wrist grip unless you nail your hand position. The slight torque post shot pushed arrow to side a bit.
The funny part is I was setting up two new bows at the same time.. one was going left, other right. Funny, right? Or funny, left?