Your 3rd question is and interesting one. This is
Post# of 32635
PR = focuses on media outlets, bloggers and public facing audiences. It is the positioning and building of a company's brand - raising awareness, moving interested parties through the sales funnel, and even sales by promoting company and it's offerings via secured media coverage. This also often includes social - the amplification of that coverage to help drive to business goals. The presentation here is how news impacts target audiences and the company's overall industry.
IR - this is the same thing, except the audience are financial audiences - the people who help stock prices rise, decrease or stabilize the price. These are often financial analysts or financial media like Bloomberg, WSJ and more (although PR works with these teams too). The presentation to this audience is based on how the news impacts the company itself - if you have a partnership, revenues are increasing which is great, or it may be that you lost business and this impacts revenue which could decrease the recommendations on your stock. This team handles earnings announcements - PR may be a part of writing or editing, but this team manages the financials and then the sharing of this information with analysts and other financial audiences.