Question 1
Does rule 17.1 apply at a leeward gate?

Answer 1
Yes. However, when approaching the gate as shown in the diagram below, Blue's proper course may be to round either mark.

Question 2
Does rule 18.4 apply at a leeward gate?

Answer 2
Yes.
When approaching the gate as in this diagram, the boats first enter the two-length zone around the starboard leeward mark (LS) and are therefore about to round Mark LS. However, Blue may have a proper course to round either the port leeward mark (LP) or Mark LS. If Blue gybes beyond her proper course to round Mark LS before she is about to round Mark LP, Blue breaks rule 18.4.
However, when the umpires decide both boats are about to round Mark LP, rule 18 ceases to apply with regard to Mark LS. Blue must now give Yellow room at Mark LP, but is not obligated to pass the mark herself. In this case, Blue breaks no rule when she gybes back toward Mark LS.