J.B.
Take it from someone who has been there - don't borrow trouble. You can imagine all kinds of wild plots and schemes and motives for her craziness but until she actually takes action, there's not point in getting worked up over what may or may not happen. If he gets served with papers and has to go to court, he goes. There's nothing you or he can do to prevent that. If his custody order says that she can't move out of state, she won't be allowed to move. If she can't afford to support herself, then your husband can petition for full custody so that she can keep her expenses down and only worry about supporting herself.
She said that she has to move due to financial reasons and wants you to take her daughter temporarily. Do that, of course, but make sure that the change in living arrangements is documented. If she makes a move quickly you probably won't be able to make it to court before that happens but you can draw up a document called a "stipulation of the parties" that is signed and notarized by both her and your husband that will list the date of the transfer of custody. When we did this for 6 months when my SD was in Kindergarten my husband continued to pay child support but her mother paid it right back to us, which was faster than trying to get the order suspended through court. We did this again when she "temporarily" moved in with us 2.5 years ago, before her mother moved out of state and disappeared. Without this notarized stipulation, having the change in custody recognized and child support order/enforcment stopped would have taken a lot longer than it did.
Courts are not as stupid as you seem to think they are. "Self imposed poverty" doesn't get support orders or custody orders changed. To me it doesn't sound like she's trying to get more support or move, it sounds like she's trying to get out of motherhood entirely. I never would have thought that my SD's mother was capable of that but, well, it's been more than 2 years and there you go. My SD's mother was a scheming, manipulative drama queen but very little of what she plotted actually came to fruition. Really, take this one day at a time and don't let your imagination get the best of you.