How do I add a Temporary Modifier to the actor value? e.g. health? I would like to simulate a fortify health potion, but can't find the method to do so with. I've tried ModAV and ForceAV, but those does not seem to work. Is there any SetBaseAV?

The easiest way would be to apply a spell to the actor using the same magic effect as a fortify health potion. The way the fortify health potion works is by applying an effect that is a "Peak Value Modifier" to health, meaning that it temporarily increases the peak/maximum value for health. If you wanted to do it via script, you could probably ModAV health by X, then damageAV health by X. To return it to normal then, you'd just ModAV again to return it to what it was before.