1- Let everyone involved in the site creation (Clients, PM, devs ) communicate right on the live site by using "sticky note"
2- It can't be use on a public site without major refactor
3- Yes it will. I have a "Mirror" feature plan so that you can even mirror issue from "stage" site to you dev
4- Still unknown yet. Probably will have a free plan with some restriction, but the "small agency" tier would be around 29/mo