Prob should have done this at the end of DF, but oh well. Maybe someone else did and knows the details of exactly how it works though…
I can’t seem to get the assassin buff to pop up, but don’t know if that’s because the character is lv21 and there’s a strict rule against fighting anyone even a single level lower. My 80s couldn’t become assassins by beating up children, that makes sense. But the lv21 earthen isn’t getting it either, even after waiting a day and rolling a second trial char in case the game doesn’t like me picking on a specific alt.
The free trial chars at 20 could get hit with both effects, completed the DF bounty achieve using them. Even got the slayer title, though that doesn’t transfer to the linked main account.
I’m more worried that trial accounts are not considered valid pvp targets though. That lv21 earthen wasn’t getting WQ credit for Extermination.
EDIT: after numerous achieves, the answer I’ve come up with is… mostly no. Free trial accounts do not grant honor for killing them, no quest credit, no becoming an Assassin. If Sliver of N’zoth ever gets fixed it won’t work for that. Direct achievement credit is inconsistent though, Frontline Slayer would not count freebie kills. Elemental Conjuror DID work. There is no hard and fast way to predict, it’s up to however individual achieves were coded.
Keeping 2 windows open and login-swapping to chars on the same account without exiting cleanly does work, it’s just a major hassle.