I'm so sorry your little girl has this problem at such a young age.
working as a surgery tech i did see this problem quite a bit, but it was mostly in cats. i believe the treatment and teh success rate of the treatment is very different in dogs then it is in cats. that medication you mentioned has been used in dogs but some say it does absolutely nothing or it can be harmful, others say it works...
i know lactulose and metamucil works in cats. if the canned pumpkin doesn't help i guess you can try the metamucil. also i'd add some yogurt for good measure. the good bacteria will help the digestive system.
there is a surgery that we performed on the cats called a subtotal colectomy, but i can't remember if this was something we did on dogs too. During this surgery the colon is basically removed. Diarrhea usualy ensues for a few months after surgery but it does get better with time. the poop will never be normal again but it will get close.
I'm sorry your puppy has this. its definitely not something fun to deal with, and unfortunately her life will be effected.