How does All Things Weaken interact with Heart of the Wildfire? When Wildfire places Presence, does it then destroy its own Presence?

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.