The best advice is to seek an attorney to help you in that matter. An attorney would need to first reading through your divorcee decree to determine what the terms are that you are seeking to enforce and if any of those terms violate public policy in this state. Generally, speaking you can enforce a valid court order from another state in this state based on the Full Faith and Credit Clause of the US Constitution.
If you are required to amend the petition and not just the proposed judgment then you will need to start all over again from the beginning and file the proper paperwork with the corrected info. If you only need to amend the proposed judgment then you will just need to resubmit the proposed judgment with the correct info.
There is not enough information to answer your questions. If you have a trial date already you should know what the trial is going to include ie...whether or not child custody is going to be an issue at trial?
If you forgot or neglected to report items in your required disclosures then you should consult an attorney privately to discuss your options. You may be able to file a new Request for Order to disclose the information and obtain a new support amount. However, the opposing party may be entitled to reopen the dissolution action because of your failure to report your required information correctly. It would be best in your situation to contact an attorney to review your documentation and alternatives.
If a response was filed by the opposing party then you will need to go to a hearing to set your case for a settlement conference and or trial. You will be required to fully disclose all of your assets and debts prior to the court making an order with or without trial. As such you should seek legal help to give you advice on the process for a contested divorce.
You and or your attorney can dispute the date of separation at the hearing or during trial. Any attorney would be able to provide you with information and assistance in proving the date of separation. I would suggest that you hire an attorney to help you and not take the date of separation lightly in your reply or objection. Feel free to contact me for a free consultation, just mention you are from avvo.com www.attorneysamwalker.com