If I gain an Element in the middle of resolving a Power, may I use that Element to gain a later threshold?

If I gain an Element in the middle of resolving a Power, may I use that Element to gain a later threshold?

Yes, you may.
This can happen, e.g., if the first part of Indomitable Claim reveals an Element on a Presence track, or if the land gets Blighted and the card is Aid from Lesser Spirits.
Likewise if you lose Elements in the middle of resolving the Power, you may lose further thresholds.
Note that any parts of the power that have already happened cannot be modified — those thresholds are either already hit or already missed. In particular, any thresholds that modify Speed or Range are checked first (before the rest of the power), and ~or~ choices are made before resolving either option.