Welcome to the site. I lived overseas for 8 years, 3 Germany and 5 in Italy so I am not offended by your European frankness.
I understand the English language doesn't always come across right when translated...I know my Italian and German probably shocked some people.
Bite inhibition is what Lili seems to be missing. Teaching her that biting hard is wrong is rather simple in and of itself if you can be consistent. My yorkie (17 months) had to learn, I taught him by saying "ouch!" when he bit too hard. If he continued to bite hard and wouldn't listen to my ouch, I would calmly pick him up and put him in his pen. He has learned to bite softer, and we are continuing the work of no biting by me saying "nice" to him. He stops, looks at me, and seems to think and stops the bite. Teddie also loved to bite the brush while I was trying to groom him. I started giving him a chew toy to bite while I was brushing him, he would leave the brush alone and stop trying to attack it. I have also given him chew treats, he is so absorbed in the treat that he ignores the brushing. We are still working on keeping him from chewing the brush on his muzzle/face area.
For the dinner, and your roomate giving your pup a taste...could you try keeping your dog in a pen while food is being eaten? That would prevent the roomate from sharing human food and also from your baby learning bad habits from the other dog in the home. If this is not an option, maybe you could move yourself to a different location to eat, away from roomate and her dog, and try to train lili to not beg for food away from other stimulus.
As far as preventing the dogs from playing together, that will be difficult. You could always work with both of them to play better together, but if you want to try and give Lili exercise without Momo around, try taking her for walks, or to a park to burn energy. Also if you have your own room you can burn her energy in the quiet of your own room.
I hope something I said helps in some way. Good luck!!