Welcome to the forums w0lfang!
1. Nothing, but that should not happen very often. (If really ever)
2.correct, if a creature has slow, and you give it the fast trait, it is normal. If you give it the fast trait yet again, it will now be fast.
3. When you cast a; equipment, enchantment, conjuration, creature, incantation, or attack(spell), You are casting a spell. The only time when you are not really "Casting" is when you are revealing an enchantment. Casting is an action, revealing is not. If you look in the glossary, at the beginning of the Rules Book. it will say "Page 16 SPELLS" and list the for mentioned above.
Hope to have been of some help. Happy Gaming!