Yes, if there is a support order in place. If so, I suggest that you ask that the order be terminated because it appears that this child is emancipated.
yup
Are you assuming you won't have custody?see links
Yes, but you need to modify. see related links
NO, not if the child is over 18 and receiving an SSI check.
A parent who is receiving public assistance (including SSI) should not be ordered to pay support.
Yes, but you need to get it modified. see link
If by "back pay" you mean retroactive support, that is typically awarded with the entry of the original order for current support,
An obligor who fails to pay child support pursuant to an order can be put in jail.
You can contact your local child support recovery unit for assistance in receiving child support. You can also take the other parent to court to enforce child support payments.
the guardian should file to have the order transferred to them as well as file on the parent currently receiving it to also pay. Or the obligor parent should file for custody.
see links
Yes. You husband is obligated to pay child support according to the order of the court and is in violation of that order if he fails to pay at any time. His employment status is not a factor in how much he owes, when he pays it, or how much of his unemployment check is left (if any) after the support payment is made.