I am sorry to hear you lost one of your babies. How is the little surviving pup doing today? I pray mom and baby are doing better. Sometimes the milk is a little slow coming in but since there is only one pup I would imagine she should get plenty of milk. The main thing is that you don't want him to wear himself out trying to nurse. Here is an interesting article that might help you some:
http://www.showpapillons.com/formula.htm
I have used the recipe in paragraph 10 on several occasions and I feel it really helps with tiny puppies. You can also give some to the momma. Make sure the momma does not get too hot or that will effect her milk production.
As for the diarrhea the mom has, if it does not clear up with the pumpkin, you might want to ask the vet what would be safe to give her.
Did the vet diagnose the puppy that died with a heart problem? I am just curious about what kind of heart problem? Was the mom in labor a long time? Was the mom given a shot of oxytocin to increase her contractions? Any input you have might be valuable for others going through a difficult labor.