Hi! Thank you for your kind words. I'm happy you like the game. Here are the answers to your questions:
1. Arcane is everything related to the occult powers of the ancients. For example, the GM could present an obstacle like an arcane barrier that protects an area (i.e., the center of a cursed church). In this case, players can try to manipulate this power (to disable the protection or to use it to their advantage). About the bullets: you can use them to stun or to parry. Check the character sheet under the "Firearm" section.
2 and 3. Harms can have different sources. I.e., a bomb can inflict FIRE damage, a magic trinket ARCANE damage, a sword PHYSICAL harm. Some creatures can inflict MENTAL harm (i.e., paralyzing a character with fear or horror). The executioner can resist this kind of harm. The priest can resist OCCULT harms (i.e., a cursed spell or weapon).
4. You can use it whenever you want, but of course, the enemy can try to stop you during combat. So, using in a dangerous situation can be difficult or put a character in a worse position.
I hope I answered all your questions. Let me know if you need more help!