A permanent can be exerted even if it’s not tapped or has already been exerted in a turn. If you exert a permanent more than once before your next untap step, each effect causing it not to untap expires during the same untap step.
July 3, 2020 · Core Set 2021
rule 701.43b not present in this release
May 3, 2019 · War of the Spark
The phrase “the [subtype] you amassed” refers to the creature you chose, whether or not it received counters.