1) The "trinket" was made a healer additional in 4.x. It is now a healer role option instead because CC was gradually added back into the game and naturally they didn't want this to be a case of Stormblood again where essentially Healer's would only had one additional to choose from because the other one would absolutely have to be purify.
2) Ye RIP traits, also gone in 4.x. They felt there was just too much for new players to learn and that's why the mode historically hasn't done well; so they pruned it down.
3) PvPaissa discord -
https://discord.gg/sJpWet ; Also, make some friends. A lot of helpful players on Mana where you're based and imo the most HQ game quality; so hopefully your Japanese is good or you know people within the feast community there, as training q is active and vets dont mind you DMing them for help (but a lot of them don't speak any English) and when you're better you'll get customs LS invites (a few people run them, just play lots of training and you'll get invited; if training q doesn't pop they're in customs or doing RW).
4) We just had a balance patch so there's no tier list. Generally at the moment everything is considered "good" - it's really hard to design these until LP teams start scrimming and even then, LP doesn't equate to solo queue, something that's strong in LP might necessarily be as good in solo q, realistically what's good and what isn't for ranked won't be shaped until a couple of weeks into the new season. Generally, I do not recommend Casters. IMO BLM is currently the best caster but it's strong relies on you already being decent enough to know what's safe and what isn't, if you play it as a newbie you'll most likely just get farmed.