>1. What are DH's responsibilities? Should he send her
>everything now or wait to see what she wants to see?
I would immediately write (do EVERYTHING IN WRITING) and ask the other parent if she wishes to continue with the support action, and if so, then please forward copies of her last 3 pay stubs, her last 3 years tax returns, and her last 3 checking account statements.
If she doesn't provide, then you have the upper hand, because you can tell the judge that she wasn't cooperating.
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>2. When does the clock start for retroactive CS in this case?
Retroactive support may be ordered for any time beginning on the date of filing of the motion to modify support. Your facts say that BM "brought up" CS change. Not sure what that means, but if the judge has determined that a request to modify support was made as part of a hearing or otherwise, then the date that the oral request was made would probably constitute the "filing" date, from which retroactive support could be ordered.