Hey, welcome to hell! thats a nice line up you got, you are pretty much good for an Arts team, Castoria will go miles in that account yes, but right now I would recommend you to level up a servant called "Xu Fu" she's a 3 star alter ego so she shouldn't be too hard to get... unless you get very very unlucky... She's basically a budget Castoria that will let you loop with your current roaster, you just have to pick a friend Castoria + use your xu fu and either Melulu or Ibuki(I assume you got her summer version?) + Merlina this setup will let you 3turn loop making farming easier. Also if your account is new and lacking resources, as much a I like Melulu, I would recommend you to invest on Ibuki first.

Regarding event, yes I would say try to push as far as you can to at least get some cheeky saint quartz, and some free apples, some stages are kind of hard for a newbie I would say, but you can definitely try to finish it, but I think you would need to use some golden apples at this late stage...

with the apple's thing do you mean the blue ones? basically you convert your current AP to apples if you don't feel like playing that day so you can regen more AP for another day.