Hi,
1-Ruminant fat it's the best. But
2-Butter it's okay. It's the least harmful thing from dairy. Lactose is out so is casein.
How much? increase gradually.
3-Idk, try.
Carnivore it's an elimination diet, extremely powerful, introduce foods one by one, and you'll see when a food causes a reaction.
I can only get pasteurized butter, so I don't consume any. I rely on pigfat and sheepfat.
IMO, the best way to get as much fat as you want is buying a sheep from a farmer and slaughter it. It's unbelievable how fatty it is. You don't have to think about how to add enough fat to your meat.