The OED considers preventive preferable, but preventative has been around since the 17th century. The syllable is available if you prefer to stretch the word out. Merriam Webster's lists it without comment.

Since both are 17th century in print, neither is prevenient.

The OED did not admit omelet (omelette) to their Dictionary until 1987. The word first appeared in English in 1846 in the "Jewish Manual, or Practical Information on Jewish and Modern Cooking." Acton, in 1846, in his "Modern Cookery," introduced the "omlette(sic) aux fines herbes" to English cooks.
All examples from the OED.