with no model of how the outside world affects the opposing moves other than the abstract assumption that the opponent will make the best move available.

A good chess AI would not be so constrained. A history of all chess games played by the particular opponent would be quite useful. As would his psychology

Additionally, the actions that a chess AI can consider and take are limited to moving pieces on a virtual chess board

Is it worth me examining the tree beyond this particular move further? How long will it take me (metacognitive awareness...) relative to my time limit?

The chess AI simply does not have any awareness of anything outside the chess game.

Unless someone gives them such awareness, which may be useful in some situations or may just seem useful to naive developers who get their hands on more GAI research than they can safely handle.