In Gokigen Naname 3D, instructions say that draw the diagonal lines. However, in the solution, both the top 2s are connecting along the line instead of diagonal?

Yes, this mistake has been pointed out before in this thread, the cells are drawn wrong in that part of the puzzle. The rules are correct.

In the instruction book for Cave, did the sample solution flip the shaded squares? The shaded squares are not supposed to be part of the cave, correct?

The puzzle solves the same either way, its just different notations.

"Shade some cells to leave behind a single, connected group (the cave), with no enclosed, shaded cells. In other words, all shaded cells must be connected by other shaded cells to an edge of the grid."

This plus the flip of what's shaded in the example led me to think that what I wasn't shading were the walls of the cave and because they were unshaded, the walls were allowed to be enclosed. I should've tried to solve the example - I would've noticed there wasn't a unique solution with that interpretation.

Going back to the competition puzzle, this is making much more sense now, thanks. :)

How do I post an image of the puzzle in the discussion forum post ???