How much damage (if any) should the Magic Cop do with their backup attack of sorcery?
I'm thinking that without the Blast schtick they shouldn't do any damage, but that leaves them without an effective backup attack. I did track down something I vaguely remembered from FS1 via the old FAQ at blewer-d.tripod.com/fsfaq.htm
Martial Arts for the backup attack is pretty straightforward then, but if the character then takes a schtick that requires a Sorcery check (such as heal object) what should the Sorcery skill be set to?
Eleven other archetypes also have no backup attack....it's not that uncommon to have no backup. Still, I doubt a MA of 12 would unbalance the game, so long as they don't get access to MA schticks, so IMO your solution works too.
I would keep it at 13, just as is written. Just give them MA as an extra. Alternately, you could give them a special schtick like the Gene Freak and Supernatural creature have, that says "you can use your [other value -1] to punch". Something like: "Never unarmed. Make hand-to-hand attacks using your Guns value-1."
I assume they have a martial arts AV of 7. The book says if you don't have a skill, it's assumed to be 7. Why wouldn't it be the same for attacks? If a magic cop wants to punch someone, he can, but he'd need to roll well.