So as the controller of the Copy Enchantment, you can apply first either Containment Priest or Copy Enchantment's replacement.
You must apply Copy Enchantment's replacement effect first, because etb copy replacement effects need to apply before mosts replacement effects.
616.1. If two or more replacement and/or prevention effects are attempting to modify the way an event affects an object or player, the affected object’s controller (or its owner if it has no controller) or the affected player chooses one to apply, following the steps listed below. If two or more players have to make these choices at the same time, choices are made in APNAP order (see rule 101.4).
616.1a If any of the replacement and/or prevention effects are self-replacement effects (see rule 614.15), one of them must be chosen. If not, proceed to rule 616.1b.
616.1b If any of the replacement and/or prevention effects would modify under whose control an object would enter the battlefield, one of them must be chosen. If not, proceed to rule 616.1c.
616.1c If any of the replacement and/or prevention effects would cause an object to become a copy of another object as it enters the battlefield, one of them must be chosen. If not, proceed to rule 616.1d.
616.1d Any of the applicable replacement and/or prevention effects may be chosen.
Scenario 1: Erebos' ability is taken into account (as told by rule 614.12) to see if Containment's priest lets him pass. Note, however, that your devotion to black is not information about the entering-the-battlefield Erebos, so you don't use rule 614.12 to calculate your devotion to black. So you calculate your devotion at the time you apply the replacement effect, just before Erebos tries to enter. In (a) and (b), your devotion to black is lower than 5 and Priest lets him pass. In (c), your devotion is 5 and Erebos is exiled.
Scenario 2: The same reasoning applies: 614.12 applies to determine whether Copy Enchantment enters as a creature, but the number of enchantments you control is determined before Copy Enchantment enters the battlefield. So, in (a), the Copy-Starfield enters the battlefield. In (b) and (c), it doesn't.
Scenario 3: In (a), Erebos tries to enter as a noncreature and gets to enter. In (b), both type-changing effects are taken into account but the effect from Erebos wins because it has a later timestamp, so Erebos enters the battlefield. In (c), only Starfield applies, so Erebos is exiled.
Nah, it lost a lot of power with the sixth edition change, and more when mana burn got removed. You're basically mana depleting yourself to reduce your opponents mana by one for a turn, and that is if they don't chose to cast an instant in response. It's an over valued card that isn't all that good.
The Sixth Edition change happened before Rishadan Port was released. The removal of mana burn actually made Rishadan Port better, because mana generated during the upkeep step could no longer be used during the draw step. Rishadan Port has always been busted.
You must apply Copy Enchantment's replacement effect first, because etb copy replacement effects need to apply before mosts replacement effects.
Scenario 2: The same reasoning applies: 614.12 applies to determine whether Copy Enchantment enters as a creature, but the number of enchantments you control is determined before Copy Enchantment enters the battlefield. So, in (a), the Copy-Starfield enters the battlefield. In (b) and (c), it doesn't.
Scenario 3: In (a), Erebos tries to enter as a noncreature and gets to enter. In (b), both type-changing effects are taken into account but the effect from Erebos wins because it has a later timestamp, so Erebos enters the battlefield. In (c), only Starfield applies, so Erebos is exiled.
https://magic.wizards.com/en/articles/archive/news/see-cards-vault-transform-2017-11-06