You have o submit your licence details twice - once when you respond confirming you were the driver and again after you have accepted the fixed penalty. Many people overlook this and see their case end up in court as you have.
There are a couple of other things you might like to consider as well.
Firstly, if the offence date was 21st May, the police must begin a prosecution by 21st November. It's probably in time but it might be worth checking the date the charge was issued.
The other thing is that it would be worth asking the court to consider sentencing you at the fixed penalty equivalent. Magistrates have the discretion to do this if they believe it is justified. Although the reason your case has gone to court is entirely your fault, you may find a kindly magistrate sees fit to do so. You should make your request under the "mitigation" section when responding to the SJPN.
As far as completing the form MC100 goes, it is (apparently) an offence to fail to provide your financial information when asked. I don't know what legislation covers that and I have never heard of anybody being prosecuted for it, but I don't see any point in not providing it. If you do not the court has the discretion to make what assumptions it thinks fit about your income though they normally simply revert to a default figure of £440pw. If you make a request to be sentenced at the fixed penalty level and it is refused, that is the figure that will be used to calculate your fine. Although the form asks for quite a lot of information, it is principally your income the court is interested in and if you simply provide that I doubt there will be any comeback.