The fact he hasn’t paid support is one point. The fact there’s been no contact in 4-5 years is equally part of it. Not that you “could” find him if you made it a campaign or pressed relatives. You may not want to track him down and establish contact. That could blow the waiver.
The Fafsa looks at the custodial parent. Not the AWOL parent’s possible assets. OP doesn’t need to report what her ex-husband has, may have, or what the BIL thinks. They’re divorced. Even on the CSS, each parent reports for her/himself, in a divorce. The absent parent would use the NCP form.
So I agree, contact a few colleges, or otherwise find their NCP waiver form, and see the sorts of questions asked. Some do require back-up letters (GC, social worker, clergy, it depends.) If support stopped and you discussed that with your attorney, maybe that attests. It’s not that you had to have filed a lawsuit.
edited to reflect ex, not “husband.”