1. Does it send a message saying "You completed your slayer task. You obtain xPoints slayer points. Please talk to Vanakka." after you complete the task and it just doesnt give exp?
2. That could possibly have to do with your model header which is client sided which I am not too familiar with
3. Again, that is client sided. I would suggest checking the RSInterface code that handles the spell book. Does it show any text in the top left corner of the client window when you hover over the spell? And can you use the spell, just the sprite is missing?
4. It is most likely cache packed so you can either open the cache using toms cache suite and manually change the cache packed interface, or you can just figure out the line id number and use sendFrame126 with the line number and text you want to replace it with.