There's no reason to have to pick an exotic crossbow proficiency; the 'slinger gets proficiencies with all firearms, after all. Just give them straight proficiencies for all crossbows. I'd also say that, ideally, the battered firearm can be replaced with a battered repeater, or masterwork variety of the standard crossbow.
But I seriously hope they update this soon; the alt looks like it would be a hoot in PFS play and at the moment that's simply not in the cards.