a) Yup! Luvdisc has a 95% encounter rate in Ambrette town (head south down the beach to get to the town's water) while fishing with an Old Rod, and will always hold a heart scale.
b) I was pretty lazy when editing the Chateau because there are just so many trainers there, so most of them just got a level increase. However, Gym Leaders and Elite Four members have full teams of six with custom moves, items, etc, and have multiple rematch teams. Their levels can be boosted further using the challenge writs, and Diantha's Chateau team starts at level 100. I still don't know if the writs can boost her levels above 100, but it was possible in BW2's challenge mode so I wouldn't be surprised if they could (if they can she'll be able to reach level 120 with a black writ of challenge).
If you're using luma you can bring up the Rosalina menu by pressing L+Down+Select, which will have an option to take screenshots. They'll be saved to luma/screenshots on your SD card, and from there you can upload them to imgur or something similar to get a shareable link to them. Thanks again for the kind words! Drayano's hacks are what got me into ROM hacking in the first place, can't wait for his Platinum hack to be finished.
EDIT: It turns out the writs of challenge actually replace any custom movesets on the chateau trainers with default level up moves, so they're not worth using if you're looking for gym leader rematches.