Re: Coffee: how bad would it be?
Methinks you already know the answer and are just posting here for reassurance, so I'll be the first (of many) to say "Don't do it".
In my opinion it will just make your life harder, as once you have had that coffee you've broken your fast and hence may as well eat other things... i.e it is not the coffee per se but what it leads to!
I find that if I let myself have a coffee, I then think well why not have a green tea, diet coke etc as they all contain approximately zero calories; that coffee just opens up a whole realm of possibilities. Whereas if I stick to water it is much easier as there are no choices to make.
Whilst fasting if I want something like coffee I always resist, saying to myself: "No, it will just make things harder if you do!"
Hope that helps, take care and best of luck,
John.