Hi S.,
Just a thought about your middle child - I am a middle child. I also have some issues around anxiety and OCD, that have nothing to do with being a middle child. However, I also fall into the pretty typical personality and situations of middle birth order.
I'm not sure how much of this will relate to your family, since you don't list examples so this is just broad advice based on my own experience - not saying you don't already to these things. It sounds like you are really trying to do what's best for all your kids so I think that is good!
For me, specifically, being the middle kid was about feeling like I was always the 'extra'. I had to go with my brother to his baseball games because she couldn't leave me alone, but I couldn't play because it was an all-boys league. I HAD to play soccer because my brother already played soccer and we could have the same practice times (didn't want to do soccer - wanted to do ballet. I only got to take ballet when my sister was old enough and we BOTH could go at the same time). I had to NOT go to a playdate because my sister was too little and couldn't go or I would have to take her with me so she didn't feel left out. I couldn't borrow my brother's toys cuz I had my own "girl" toys, but I had to share a room with my sister and had to 'share' my toys because she was my sister and we were both girls. etc etc Yep - in retrospect it's all whiny pouty baby stuff. But it REALLY makes an impact on kids when they are younger.
So I would say make sure your middle kid has a life that is all her own. Let her participate in whatever extra curricular activity SHE wants - not just what is convenient for you to get her to because of what your oldest participates in. Let her NOT share certain toys that she loves, but she absolutely should be a good 'family member' and share or play with her siblings sometimes too. I think it's a balancing act.
We dealt with some different mental health issues than in your family- my dad was an alcoholic and drug user who was pretty unstable and would come and go as he pleased with whatever women he pleased. I responded by needing a higher level of control over my own environment. To this day I don't like surprises. So, I can see how a kid who already may feel like they don't have much for themselves may be responding similarly to a sister who is taking up even more time - I mean your middle daughter is 12 - which is the height of "the world only exists for me" pre-teen angst. So, maybe you could ramp up conversation and details with her, instead of NOT giving her information. For example you said "little does she know that we have a party planned for her". Stuff like that would drive me CRAZY as a kid - I really wanted information and I would have spent the whole time before my birthday feeling forgotten. Then I would have felt guilty because I felt that way at the surprise party - and really neither set of emotions is what you want for your kid. Maybe you try to give her some extra 'control' over her environment - let her pick the veggies for dinner or have her be your 'helper' who makes the salad and gets to decide if you add tomatoes or green peppers. That kind of stuff is really helpful for my nephew who has some control issues of his own.
Good luck with your youngest - that is scary and I wish you all the best.