I think you are right as to who has he bigger and more winnable claim against the couple who took and spent the GoFudMe money - the homeless man or the donors, and it’s likely the donors. Maybe they should get together and make a class action suit on their own behalf, with the proceeds (if they win) going to the homeless man.
Something else to think about is that the IRS is probably all over these cases, too. Someone who gets $400,000 — or even just $10,0000 — in “donations” from total strangers would seem to have a tax liability here.