Does the warpriest's ability to spontaneously cast cure spells mean that he can use fervor to cast a cure spell on himself without having prepared one? Having the option to expend a spell slot for slightly more healing than fervor provides by itself could be useful in certain situations.
It says he can use fervor "to cast any one warpriest spell he has prepared" without reference to or exception for spontaneous casting, and spontaneous casting doesn't have general enough language to let it be used just anywhere you can cast a spell. So no, by RAW.
But IMHO it would be a perfectly reasonable houserule to allow it.
No, the fervor ability specifically says the spell cast must be one you have prepared.
Spontaneous healing specifically says the healing spells are ones he did not prepare ahead of time. He is sacrificing the energy from a prepared spell to cast an unprepared one but it doesn't change the unprepared spell into a prepared one so no fervor.