You can try reintroducing the kittens to your neighbors momma cat again. Take a towel and rub it all over mamma and her babies, then rub the same towel all over your orphans to transfer the scent. Scent more than sight leads to rejection.
If they are not taking a bottle but eating out of a dish at that age I am amazed. It is hard to get a kitten to take a bottle. I raised an orphan of that age once and I wrapped him in a towel and laid him on his back to get him to take the bottle. Once he got the idea he readily took it but my hands were covered with teeny tiny scratches from his little claws hanging on while nursing. And at that age I did still stimulate him to go to the bathroom by rubbing a soft slightly damp washcloth on his rear. You are not kidding that it is hard work. But if you don't have to bottle feed them you are doing good, would be best though if you could get the neighbor momma cat to adopt though. And yes your are correct that the bathroom issues were from the sudden change in diet but now that you have them on kitten formula it should be ok in about a day or so. They should not be making a firm stool at this age anyway. Good luck.