What should we remind Hashem?
Rashi: All payment that I 'owe' to you and your fathers.
Radak: If you will say that I forgot your merits, and your sins are not many, like I say that they are, remind Me, like one who reminds his friend of something that he forgot.
Malbim: I will not mention your Chata'im. If you have any merit and claim against Me for your benefit, mention it to Me!
What is "Nishafta Yachad"?
Rashi: We will come for judgment.
Radak: We will judge whether I struck you according to your sins, or if I erased them for you.
Malbim: We will see if there is substance in your words.
Why does it say "Saper Atah Lema'an Titzdak"?
Radak: First tell your claims, so you will be vindicated - "Tzadik ha'Rishon b'Rivo" (Mishlei 18:7). I.e. even if you will tell [first], if you will tell the truth, you will not be vindicated, even if you do tell all your sins, and do not tell all the Tzedakah that I did with you.
Malbim: I will not dispute with you at all! Just tell all that you want, in order to be vindicated. I want you to be vindicated. Even if I know to prove opposite to your words I will be silent, as long as there is some claim.