You just sign it by typing your full name. You are not represented. You are defending it yourself as a layperson. so don't tick that box. Just tick the "Defendant" box.
E1... Obviously, it will be you, the defendant, that is attending the mediation call, unless you want someone else to take it for you. It is not part of the judicial process and no judge is involved. Your next door neighbour or anyone you want can take the call. Just make sure that you tell them to offer £0 and not discuss anything else.
I'm sure that everything else is self explanatory.
I've already explained how to send the form.