Firstly, and taking what you've posted at face value, you have my sympathies: I've seen FCC inspectors in action and some of them are nasty bullies. They have a reputation for preying on women travelling alone as well.
Next, I think LM might be overstating things. The rules for carnets require "permanent ink" to be used. So long as you did actually write the correct date in pen, I think FCC would have a hard time prosecuting you on the basis that you used the wrong type of pen. This is especially the case since, as others have pointed out, the tickets aren't easy to write on.
As to your address, it's not entirely clear from what you say, but I think there's a key difference between you mistakenly giving the wrong address and then correcting it once you realise your mistake and you giving the wrong address but only correcting when challenged.
If you are sure that your version of events is correct, then you shouldn't be coerced by FCC into pleading guitly or trying to settle. It's a decison only you can make and you should really get proper legal advice. You've got to weigh up the risk of being found guilty against the likelihood that the Court will find you to be credible and honest, in which case I'd think it highly unlikely it would convict you for using wrong type of pen.
Also bear in mind that it is easy for FCC to send out letters threatening prosecution and they probably have not reviewed the matter in any great detail at this stage. If you - having taken legal advice - write back to them saying that you intend to plead not guilty and defend the charges, they may review the matter again and reconsider.
