Maker's Muse description, Synthesis spelled Syntheses, And false descriptions.

[Eemeefu]

Just as title says, the word synthesis is spelled wrong in its description of what it does.

Could it be they're using the plural form?

Could be, but I noticed the other ability descriptions didn't have it spelled that way.

As Obsy mentioned, this is indeed the plural form. Instances where the singular form is used in description text for commands either refer to the "current synthesis" specifically, or have the noun being used adjectivally, such as in "synthesis progress."

Temporarily reduces chances of success to improve quality. Most effective for decorations.

In the above example, the benefit would be the improved quality resulting from successful actions executed while the Inspiration effect is in place. I can understand how the current phrasing could be somewhat confusing, and so I've slightly reworded it for clarity's sake. The change will go live in the next patch release. However, I am unable to locate any of the other multiple instances you mentioned. If you have any more specific examples, please let us know. Thanks!