1. Griphook bejeweling his paramour seemed like a dead-cert aspect of goblin mating rituals!
2. I felt so sorry for writing Arthur so utterly inept and clueless, to be honest. But on the other hand, he was supposed to be completely lost, so...
3. I had first planned to write two separate stories, but Bill kept popping up in the Muriel story and the Arthur story, and so I put them together, with him as the link.
4. Hee! I imagine he marries some female member of the Shadow Cabinet or suchlike, whom he met whilst working for the Prime Minister in HBP.
5. She gets to keep the tiara, because Griphook is a little perv who a) enjoys her wearing the tiara and b) likes the idea of watching Bill and Fleur handing over the tiara after Muriel's death.