bump, OP how is Bella doing today? Just because the vet hadn't heard of whelping pudding doesn't mean you shouldn't use it. Yorkie breeders who have had years and years of experience use it. I'm glad to see your DH's mom will assist you but as you know boxers are not yorkies, and I've read so, SO many times where the poster loses a puppy or two...or the mother. A 4.5 lb female giving birth is a big risk. She may be too small to pass the pups naturally. Be ready for an emergency C-Section. I really do not want you to lose your sweet girl.
I would think about the suggestion Wylie's Mom posted. If you truly love Bella and want her to be healthy, having her spayed now may be the best thing for everyone. What kind of health problems does Bella have? Assuming you haven't done any kind of genetic testing done, the puppies may be born with congenital diseases that will end up costing their future owners lots of money and heartbreak. Good luck virtualgirl, I hope you come back and keep us updated here, there are so many experienced breeders here on YT and their advice is invaluable!!!
I also hope you did not feel attacked in my post, I promise I just want the best for you and Bella!
I'm praying for her.