Parent sponsorship appeal for H&C - Any tips for explanation to IAD and Ministry

[ffayyca]

Hi, my family sponsorship application for my parents were rejected because we missed the requesting document email sent to my parent's old mailbox. We submitted an appeal to IAD and they asked for explanation of what happened to them and Minister's counsel. We talked to a few lawyers and they said H&C reason is a possibility, so what I want to know is

1. how much of a chance do I have for H&C considerations?
2. I want to write the letter myself, what information should I provide to have a better shot for the appeal?

Thanks in advance.

Frank