1. You need to ID until you hit 20 flashes in each type. I would aim for 23 just to be on the safe side just to make sure you do not miscount them. Different types will grow at different rates. When I got my legendary, charm tended to grow the quickest. You can also supplement card ID with fortune telling. Just tally any lashes from fortune telling.
If you want to be safe go for the whole 1700 cards, you will get plenty of SCs anyways.
2. Spinel has a ranged attack that is good for farming low level items. Other than that I do not notice much.
3. It took me 12+ hours to use up about 1000 cards through card ID; I honestly lost track of time. It was very tedious. I advise you have at least 1000 cards before you start and another character ready to farm if you need to.