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Yes. When an artifact is activated somewhere off the artifact pile (a Temple, Charms on a Magician, Unholy Writ on High Priest's Palace, etc), you're still able to activate an artifact on the pile itself. At this point, it's, as far as I know, possible to have six artifacts active at one time, though that number may be higher.
9 actually. Iron Pan on Zeke. Charms on a magician. Writ in High priests palace. Book of the cov + 2 covs in a temple. A Gold curse on Pithom and another on Chorazin (albiet for one turn). And one in the art pile.
Priestly Breastplate on a priest.
It actually can be used to boost the number by one more. Use Priestly Breastplate for your artifact per turn and then attack and use Meal in Emmaus to activate a final skill-shot Artifact.
Yes, but the placing is an instant ability that completed in the previous phase.
But you wouldn't want to waste space by putting in such useless artifacts as Cross Beams, ect.