::*QM and relativity: Ok 
 ::*GR in RQM: At the level of laws of nature (other than GR) formulated as a SR relativistically invariant differential equation it is that simple. It has "almost" postulate status in GR. This is exactly how the EM field equations are treated in GR. See for instance MTW or (better!) L&L; Vol 2. It is a cookbook recipe to turn a valid SR equation to one valid in a GR background. 
 ::*Schro and Heisenberg pictures: Ok 
 ::*ψ: Either wavefunction and/or field sounds perfectly ok. Preferably use both to avoid linguistic monotonicity. Probability amplitude is best avoided except for the special cases where it is possible to use it. 
 ::*The multiple time dimensions is in Penrose's book Road to reality, is Penrose unreliable? Nevertheless, that segment can/should be trimmed. 
 ::::Well, that's a reference at least. I haven't read Penroses book, but I have gathered that it is not a scientific publication or a textbook. It is more popular science and speculation with an advanced mathematical twist to it. Is the expression for a relativistic multiparticle wavefunction taken from that book? In QFT you certainly deal with multiparticle states (represented by wavefunctions) with a single time coordinate. I have never seen anything like the expression in the article. 
 ::*Energy levels: The KG equation according to Abers's book on QM doesn't predict the energy levels accurately. The equation has nothing to do with spin so fine structure cannot be explained. Why is this wrong? 
 ::::You would get the same error if you applied the Dirac equation to a boson. The KG equation is the correct equation for spin-0 particles. It follows in the same way the Dirac equation, the Rarita-Schwinger equation, and the and the Bargmann-Wigner equations follow from Lorentz invariance. 
 ::*Negative energies: This is mentioned in the article, although the solutions to the KG and Dirac eqns have yet to be expanded on to illuminate the context of negative energies. 
 ::::Agreed. Within RQM one may solve the problem by the mentioned "Dirac Sea"/ Hole Theory. A full explanation is available only in QFT. Hole theory gives correct results (mostly) but the physical interpretation is wrong; at least nobody believes in it anymore.