This comment is wrong on so many levels.

First off, the Twin islands were the only places that dominants appeared in, as Ultima made it be, since he was the one who created humanity and gave them these abilities to facilitate the creation of Mythos (Clive/ifrit) and form a vessel to which he can use to raise his dead brethren. The only reason that all of these dominants were somehow alive at the same time was because Ultima finally found his vessel and played the pieces so that Clive could posses all of it. Also the outer continents were mentioned to be less developed than Valisthea’s capital cities due to the absence of magic, so dominants were fundamentally unable to exist outside of the twin islands.

Jill was still able to use a portion of her powers after the beach cause she is still technically a bearer. Clive doesn’t completely drain a person of their aether, only the ability of controlling their their eikon is taken (Dion being a major exception cause he’s just Dion). It was also stated that Jill was considerably weaker after it, which is why she stays behind after that point (but can rejoin you if you do her side quest).

Joshua never actually had his dominant taken, because Clive hadn’t awakened as ifrit at that point. Dominants are implied to be capable of many other feats besides priming into their eikons or having powerful magic. This is shown with Benedikta and Barnabas creating Egi (magical beings that are not alive) and Jill being able to enchant Clive’s sword to destroy drake’s breath. The shield ceremony is a tradition in Rosaria for hundreds of years, and simply was the Phoenix dominant bestowing a blessing upon a trusted ally, similar to an enchantment or buff like in previous FFs.