User blog comment:Silver-Haired Seireitou/Sake Time with Sei - Sternritter and their Powers/@comment-1493741-20140327083602

While I agree with what you've said, I'd also say that its more than willpower that's required, and that Yhwach's granting of a Holy Letter/Schrift is a necessary component that allows them to manipulate Reishi in such a way as to achieve the effects that there powers show off.