I'm wondering: what if both theses are compatible? That Kindred is Harry Osborn, but not only Harry Osborn. Why he said in TASM #37 about Norman Osborn - He thinks he knows who I am. What I am"? Maybe there are no two Harry Osborn (pre-OMD and post-OMD). Maybe explaination of his return from the death by Dan Slott and revelation about him being Kindred by Nick Spencer are both true.
Let's see. Harry got ressurected by Goblin formula, but in the moment of his clinical death something went into his body from Hell (pre-OMD Peter - part of his soul?). It used him as a portal to the world of living and Harry became a vessel of this demonic force. It was growing in him like a parasite, becoming stronger and stronger until it was strong enough to completely overpower him (transform him into the current form) and began its vendetta against Spider-Man. That's why we didn't see Harry Osborn in the Nick Spencer's run.
What if Harry Osborn is a puppet/host of Kindred original identity? What if his reflection in the mirror in TASM #50 is a symbol of his slavery to this evil force, a proof that Kindred is now in the full control and Harry Osborn is a prisoner of his own body and his soul is trapped on "the other side".
So technically Kindred is Harry Osborn but that's not the whole truth. He is something more, hybrid and therefore the amalgamation of a demon (pre-OMD Peter?) and Norman Osborn's son, which would explain his strong connection to Spider-Man and Green Goblin. His thoughts are mixed by those two presences.
I believe this theory could pay respect to works by other Spider-Man's creators. That Harry really "died" as a Peter Parker's friend, that he didn't went to Hell being classified as a bad person, that his "ressurection" by Goblin formula and Mysterio is valid, that his latest adventures (BND, relationship with Menace, American Son, becoming a father of a second son, still being a friend of Parker, working in Parker Industries and Alchemax) are his experiences, not a clone, a fake or other doppelganger.
I believe also that this theory could make a bigger revelation than Kindred is vengeful Harry Osborn. From TASM #30-31 most readers got convinced that Harry Osborn is Kindred and for them it was the obvious solution to his identity. So, yeah, they guessed right, but that's no whole story and twists and shocking revelations are still in waiting
Not mentioning OMD connection and theory about a fragment of Peter's soul, which I, Spidey_62, Hugo Strange and Kingkalamari still believed in. I suppose