"Starlight deflects as it grazes the sun, as the sun's mass warps space...so the vector momentum of a photon is altered by its encounter with the sun. But an altered momentum means that a force was exerted. So, does the starlight tug ever-so-slightly on the sun? Photons lack mass, so I suspect they don't warp spacetime..."
The last sentence in this question illustrates disadvantages of the current fashion to consider ONLY invariant characteristics of an object as something worth considering. Such view applied straightforwardly to the described situation might indeed tempt one to think that a photon does not warp spacetime since its rest mass is zero. Such conclusion would be wrong for two reasons. First, in case of a photon, its rest mass does not store any information about this photon whatsoever (except for the trivial notion that it moves with the invariant speed.) It is its RELATIVISTIC MASS (its energy) that is relevant characteristic. Second, the spacetime is warped by RELATIVISTIC MASS (more accurately, by energy-momentum) of an object rather than by its rest mass alone. Neglecting this distinction is OK in case of a massive stationary object, but not OK for a moving object, especially such as a photon. A photon does warp spacetime, and the resulting spacetime curvature is entirely determined by its relativistic mass.
"In any event, this boils down to "if the sun exerts a force (momentum alteration) on a photon does the photon do the same to the sun?" My general relativity is so very poor- I suspect the answer may lie there."
The answer to this question is yes, although its details may be rather subtle. If you have two electric charges - one stationary and the other passing by, they exert forces on one another, but these forces do not generally satisfy the requirement of Newton's third law (equal in magnitude and opposite in direction); this may appear to contradict conservation of the net momentum, but it does not, because the net momentum includes also the momentum of the EM field of the system, which was left out when we considered the charges only.
The same reasoning applies to masses and gravitational field of a system.