Rule 18 "Mark Room" was not really central to the original question, but nevertheless seems to be what people want to discuss, so here goes.

What's important when approaching a mark is the relationship between two boats when the one closer to the mark reaches the "zone" (typically 3 of her boat lengths).

If the boats are overlapped, then rule 18.2(b) applies and the outside boat must thereafter give the inside boat "mark-room". If the boat closer to the mark is clear ahead, then rule 18.2(b) applies and the clear astern boat must thereafter give her "mark-room". If the boat closer to the mark is clear astern (an unusual situation), then rule 18.2(a) applies, and the outside boat must give the inside boat "mark-room" when they are overlapped.

Rule 18 applies even at a gate mark that the boats choose not to round. It is still a mark that they have to pass on the same side. Usually however, it doesn't come into play because some other rule (such as 10 or 11) enters in.

The terms "tactical rounding" and "seamanlike rounding" are simplifications - and sometimes oversimplifications. When a boat entitled to mark-room is the give-way boat, then the right-of-way boat need only give the minimum room necessary. When the boat entitled to mark room also has right-of-way, then she may take more room and the give-way boat must keep clear. The only limits are those imposed by other rules, such as 16 "Changing Course", 17 "Proper Course", and 18.4 "Gybing".

I hope that helps,
Eric