I'm actually going to push back on the interpretation canonized by this thread, that Mount Doom only activates once a player steps on it. I can appreciate the historical effort in squaring the circle, but I think that besides being unintuitive in effect, it's not actually based on a solid and consistent rule.
In the previously quoted Active section, we have this, including the example right after:
Sites are always active. A site's game text cannot be used unless the fellowship is there, although some cards may copy and use that game text.
If the game text of a site has a Shadow special ability, you may use that special ability only when the active fellowship is at that site and you are a Shadow player.
Section on "game text" :
Game text includes all the text in the box below the card type line. . .Any boldfaced keyword that appears in this box (such as Easterling, Fierce, or Defender +1) is game text.
So: keywords (as game text) cannot be "used" unless the fellowship is at that site. "Use" is not defined, but if by "use" it means "spot" (which is about all you can do with unloaded keywords), then this section completely breaks Lost in the Woods and Brutality and a small host of other Shadow cards from sets 11+ that spot keywords on sites.
Note that this does not say "once a fellowship steps on a site, the keywords are now permanently active", which is approximately what would be required for keywords to remain in force for LITW to be able to spot once a fellowship has passed it. If that line is taken seriously, then LITW etc can only spot at most 1 forest, the one that (might) be the site that the fellowship is currently at.
But let's take this community ruling and run with it. Even if the above issue wasn't in play, the MD ruling does not distinguish between pending sites that are "in front" of the fellowships vs pending sites "behind" the fellowships: if one were to use Traveled Leader to put a forest site behind all fellowships, it would result in a permanently unspottable forest, since no one could ever step on it to activate its game text. I doubt however that anyone ever has decided to track un-initialized forests to make sure LITW counts one less than one would otherwise assume by counting the keywords on the adventure path.
So what's broken here? Frankly, that section of the Active entry. It is out of date, and was never updated to reflect the realities of the Shadows cards that it would otherwise alter the behavior of. !Never mind the release of a site from Hunters era, long after rules maintenance was a job nobody had anymore.
If that section is broken, then the community ruling based on it is bunk, too. It can't actually be consistently applied without breaking about a dozen other cards, as shown above.
The most intuitive reading of Mount Doom is simple: as soon as it's played, the second half of its game text activates, the same way that everyone treats the first half (the keywords) as being spottable (and thus, well, active). I agree that in a perfect world, this would be explicit: "When you play Mount Doom, for the rest of the game, all sites in this region cannot be replaced". But sites have limited space, and clarity was often sacrificed for brevity's sake at all stages of the game's history, never mind Hunters block.
It's the world's most softball ruling request possible, but the sheer timidity of TLHH to ever add to Decipher's canon got in the way, and instead now we have a rules-lawyering unwritten ruling that has to be explained to every new player who trips on it and (justifiably) reports a bug. After all, who would have ever guessed that "cannot" means "can, actually"? Walks like a bug, and quacks like a bug.
I would propose issuing an errata to the Active section as follows:
Sites and their keywords are always active, and so can always be spotted (even if controlled in a player's support area). Other game text on sites is inactive by default unless the fellowship is currently at that site, and so can only trigger or be activated during that time. Some cards may copy and use a site's game text independently of whether that game text is currently active (see Uruviel, Maid of Lorien (1C67)).
If the game text of a site has a Shadow special ability, you may use that special ability only when the active fellowship is at that site and you are a Shadow player.
Exception: Site text is not active when the starting fellowships are played.
If a site's game text specifies a trigger relating to the state of the site itself, the trigger still activates even if the fellowship is absent.
Nurn (17U148) reads: While you control this site, each of your hunter minions is strength +1. This trigger activates as soon as the site is controlled, even though no fellowship is currently at it.
If a site modifies whether it or another site can be replaced and does not otherwise specify a trigger, that game text activates as soon as that site enters play on the adventure path.
Mount Doom (15R193) reads: Until the !end of the game, sites in this region cannot be replaced. This game text activates as soon as it is played, preventing it or other sites within its region from being replaced, even if no player has yet arrived at that site. (This is a reversal of a long-standing community ruling.)
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