How does All Things Weaken interact with Heart of the Wildfire? When Wildfire places Presence, does it then destroy its own Presence?
No. All Things Weaken modifies how Presence destruction from Blight works, but doesn't change the source of the Blight, so Wildfire is still protected from Blight added due Spirit actions. This also applies to the portion which reads "...and 1 Presence in an adjacent land": if you choose Wildfire's Presence as the 1 Presence to destroy, it will survive if the Blight was added due to a Spirit Action.
While Volcano Looming High isn't protected from Blight added by Spirit Actions in general, it does have similar protection from Blight added by its Explosive Eruption Innate Power. In that one particular case, all of the above applies to Volcano Looming High as well. Tags: Actions, Blight, Blazing Presence (Heart of the Wildfire), All Things Weaken, 3 more...
For instance, Gift of Living Energy (a Minor Power) wouldn't count it towards "If you have at least 2 Sacred Sites...", nor would Fear of the Unseen (a Fear Card) treat it as a Sacred Site for scaring away Invaders.
Note that if you Entwine with River Surges in Sunlight, their River's Domain Special Rule only applies to them (read it as "counts as your Sacred Site"). So for targeting, they would treat any Wetland where either of you have Presence as a Sacred Site, but you would still need 2+ Presence as usual. Tags: Sacred Sites, Presence, Entwined Power, River's Domain (River Surges in Sunlight), 6 more...
How does Reach from the Infinite Darkness work with Presence from Spirits with restrictions on where they can move Presence?
Since Reach from the Infinite Darkness ignores these restrictions, Presence from those Spirits can be abducted without any issue. However, that Presence can only escape to lands that it could normally be moved to, whether Coastal lands (Ocean’s Hungry Grasp), Inland lands (Lure of the Deep Wilderness), or Mountains (Volcano Looming High). If Breath of Darkness Down Your Spine doesn’t have Presence or its Incarna in any legal lands, you must choose to let a different piece escape or lose the game.
As of 2020, the only published Spirits for whom getting around land-type restrictions is relevant are Ocean's Hungry Grasp (base game, cannot normally have Presence in inland lands), Lure of the Deep Wilderness (Jagged Earth, cannot normally have Presence in coastal lands), and Volcano Looming High (Jagged Earth, can normally only have Presence in Mountains). (Discussion elsewhere) Tags: Presence, Indomitable Claim, Serpent Slumbering Beneath the Island, Ocean's Hungry Grasp, 2 more...
When placing Wildfire's Presence, Finder's action itself did not cause any destruction. It "directly triggers" the destruction (via Wildfire's special rule), but it did so by adding Presence, not by moving Dahan or Invaders. (Earlier printings of Wildfire incorrectly said "when" instead of "after" for the Special Rule; see the errata / clarifications on triggered actions.)