Let's back up a bit. The definitions do not impose obligations on a boat - the numbered rules do that. The definitions are simply explanations of what the terms used specifically mean. A boat cannot break the rule "mark-room", and the definition itself does not impose restrictions on a boat's actions.

Rule 18 imposes the obligation on one boat to give another boat mark-room. Rule 18.4 also restricts an inside overlapped right-of-way from sailing beyond her proper course before gybing. There is no corollary rule about tacking.

"Mark-room" does not entitle a boat to sail off the course, but it does not prohibit a boat from doing so either. It is not "mark-room" that prevents a boat to leeward (or ahead) from tacking, it is rule 13. Once that boat passes head-to-wind, she must keep clear of the other boat. If she can't keep clear, then she can't tack. She may turn head-to-wind, but not beyond.

No rule restricts a boat from sailing beyond her proper course before tacking, whether at or away from a mark. A boat in a controlling position may drive another boat beyond the mark, out of the zone, and all the way to the edge of the racecourse to gain a tactical advantage.

I hope that helps,
Eric