Two questions:
First: If an ex-inquisitor accepts the Heretic archetype after "falling", does he lose any of his class abilities/spells?
Second: If you begin with the Heretic archetype, are you ever able to become an ex-inquisitor?
Thanks
The inquisitor loses whatever gets replaced by the archetype abilities. Everything else stays.
Technically the Heretic could fall again, since there is nothing prohibiting it. How this could happen would depend on context. I could see an inquisitor of a good aligned god pushing it a little too hard and ending up as a heretic for their excessively zealous ways, continuing on for a while doing their thing until they cross the moral event horizon and then fall for good. It's all up to the GM though, so...*shrugs*. It's possible, at any rate.