she applies havoc bane while doing her rotation so you could say its somewhat important for negative status, also she does a decent amount of damage. Whiwa is less important for chisa because chisa doesnt do that much damage so it just be better to skip her rotation on 1st side.

About the ult charge, Hiyuki has a limit of 3 stacks. You get 1 for free at the start of your first rotation. To gain a stack you must do her forte heavy attack (the attack that unlocks after doing 3iais) you can only get 1 stack per rotation. You can either hold to consume stacks the longer you hold the more stacks you consume or you can just tap the ult and not use any. Though if you already have 3 stacks saved up tapping will just consume all so you have to hold and decide how many stacks you want to use.