Rules Questions

[The Giant]

Player A on left dead-end of a full 8-room first level finds the stairs down, Then some players move down there and to the left. At this point, player C on the right edge of the first level uses a screw this card to find another set of stairs. Some players move down and to the right. Is this legal?

Yes. Remember that there can only be 8 rooms per level, though, so it is probable that these two sub-levels will not meet in the middle, ever. On the other hand, if someone explores the middle area and there is a small enough gap, they WILL meet.

If so, what is the range between the two halves of the second level?

The range is "Not Applicable", because you cannot measure range through unexplored areas.

Can attacks be made through the "void" and does support work from one side to another?

Attacks cannot be made, but Support (and assisting in battle) both function normally.

Additionally, if a set of stairs down to the third level is found on one of the halves, does the other one become a dead-end branch with no way down?

Yes, barring another special situation.