1. Technically it doesnt have to be mushrooms/P wood or the mimic from the lab as i believe you can dye your hair in Rins bedroom which she will reach out so you know when you unlock it and it takes the orbs you collect i wont say what ones but its a culmination
2. You can revisit but from what i found you have to not divert of the axis of left then up you have to follow the direct line and i think the time of day along with weather has an effect but this could be me making a placebo out of it but yeah the diary hints at how to get there stating important points and to not digress from them.