What you say is more or less true but I feel that all of what you say can be easily explained by Jill's insecurity, she's also not the most mature person when it comes to relationships since her experience with that is "zero". Of course any compassionate and would understand when you have prior commitments and want to take care of a sick friend who feels really bad. It's easy to postpone picnics or dates, there's always tomorrow, you can't cancel a cold/sickness though and to demand that you cancel your meeting with the sick friend so you can instead have a picnic now and not tomorrow or any other day is immature and selfish. That, to me, was easily the silliest plot point in Jill's arc. That alone could be reason enough for the MC to ditch her entirely, you don't want to date narcissists or extremely immature people who don't understand the concept of compassion and commitment to other people who need help.
However, overall Jill is a sweet character, but she has her serious flaws, she's far from perfect but she's ultimately likable. She needs to learn quite a few things about life, she needs to become more self-assured and understand that even though she may be the MC's girlfriend or potential girlfriend she cannot demand that he lets his friends down when they need help - just so she can spend some time with him. She wouldn't want a boyfriend who is like that, would she?
I feel this is part of Jill's arc. Ultimately I feel you judge too harshly here. Now, she's nowhere near my favorite character, there are probably half a dozen or so that I like a lot more (Maya, Josy, Riona, Sage, even Nicole) but Jill is not a bad person, just terrible immature and insecure about certain things, probably due to a very sheltered life with few experiences. The MC is the one who will help her mature into an adult, at least I hope so... when you are on the Jill path, that is. I am not really, I played her route to see what it's like and it can be sweet and touching (the recital etc), but I find the aforementioned Maya, Josy, Riona, Sage etc much more compelling and fun overall and they are my #1 choices. They also look better than Jill who still looks like some kind of alien to me