[...]

Juan R. has been shown mathematically rigorous derivations of the
Newtonian limit of general relativity.  He doesn't like them because
they require a choice of coordinates to get the standard form of
Newton's equations.  (General relativity is generally covariant,
while the form of Newton's equations that he likes isn't, but he
apparently believes that taking the limit c->infinity should
magically pick out a coordinate system.)

Juan R. uses a coordinate x^0=ct, where t is the Newtonian time,
and thinks this makes sense even in the limit c->infinity.  Of
course, in this limit, t=x^0/c->0 for every finite value of x^0,
and all derivatives with respect to x^0 go to zero.  He has
been told how to take the limit properly, even in his coordinates
(start with a large but finite value of c, multiply by an appropriate
power of c so that the c->infinity limit makes sense, and then take
the limit), and has been shown how this process gives the correct
Newtonian limit, but he doesn't like that, either.

Juan R. has apparently casually read a paragraph or two about
Cartan's formulation of Newtonian gravity as a spacetime theory
with a preferred time, and has misinterpreted what he read.  In
particular, it is an easy calculation that in the Cartan formalism,
the spatial curvature at a fixed time is zero, but the spacetime
curvature is not; he has made the beginner's mistake of confusing
spatial and spacetime curvature (much as, in the post I'm replying
to here, he seems to confuse the scalar curvature with the curvature
tensor).  See, for example, J. Christian, arxiv.org/abs/gr-qc/9701013.

Juan R. does not understand the role of boundary conditions.  In
particular, he thinks that the need to impose boundary conditions
to obtain a Newtonian potential is somehow "unphysical" (basing this
largely, it seems, on an out-of-context quote of Christian).  This
is again apparently related to his belief that the Newtonian limit
of a generally covariant theory should magically produce the right
coordinate system.

Juan R. does not think that the solution of the Poisson equation is
really the Newtonian potential.  He also thinks that the Minkowski
metric should apply even to Newtonian gravity (!).

And, of course, Juan R. believes that his brilliant insights about
very elementary general relativity have somehow been missed by all
of the physicists who have worked on the subject for the last 90 years.