Is My Child Normal?

As parents we all undoubtedly want what is best for our children.  We hope and pray and do everything in our power to make sure they are happy, well fed, educated and healthy in every sense of the word.  I have 4 children.  My two oldest teenagers and my two youngest are toddlers.  I am aware of the different personalities each child possesses.  I am also aware that each has their own experiences in life and will accomplish different milestones at their own time.  
So that's what this post is about... Social and Emotional Development.  How do you know when your child is a late bloomer or when it's time to take action instead of sitting back and hoping they "catch up"?   Every child is different, I know this, but I am only concerned with the behavior I've noticed in my own child.  I tried talking to her new pediatrician about this but I do not feel she is concerned enough because she has only just met Bella.  I called her previous doctor and she advised me to speak with a Developmental Pediatrician.  I made the appointment so now it's just a waiting game. 

To help me out with this it might help if you knew a little bit more about my little girl.

Bella was born in December of 2009 via C-Section due to Cord Prolapse.  She scored a 9 on her APGAR and never missed any major milestones.  It wasn't until she was about 14 months old that I started to get a bit concerned about the amount of words she was saying.   She would say "momma, dadda, ball, outside and maybe 2 more words".  I knew it wasn't a major delay but because I was pregnant I was worried that if this didn't get better I would be too "busy" with the other baby to give this issue the attention it demanded so I moved forward with having her evaluated.  Sure enough, Bella tested below average and immediately started receiving speech therapy 1x week.  We were given the tools we needed to help her catch up and over 1 year she improved but still not where she should have been.  I tested her hearing despite everyone telling me it wasn't necessary and discovered she had partial hearing loss in her left ear.  It might have been a temporary loss, but nonetheless, it was definitely affecting the way she was hearing and speaking.  We also noticed her oral motor skills were off.  She has a difficult time using her tongue to pronounce certain sounds thus making it difficult for us to understand what she is trying to say.  
Speech therapy ended in Chicago the same week we moved to St. Louis.  We did not continue out here because it was too close to her 3rd birthday and they do not offer the service at that age.  It is from that age forward that the child is put in Special Ed classes in pre-school.  Soon after we moved here we noticed a dramatic increase in her vocabulary.  It was great.  She was putting together 4-5 word sentences where as before she was only stringing together maybe 2-3 words.   
In the last few weeks I have noticed some of her words are starting to sound garbled again.  Her doctor does not see fluid in her ears but we still have to go in for another hearing test.  

Here is a list of the social/emotional concerns I am having:

• Biting younger sister (In her defense, sister bites her and pulls her hair)
• Pushes, kicks and hits younger sister and dog for no reason.  When I reprimand her she just stares at me with a blank look.
• She refuses to potty train
• Hates having her diaper changed (more often when pooped.  Shame?)
• Her tantrums are getting really out of control.  She will scream, kick, pull her own hair and shake her head.  These fits can go on for 2 hours.
• She is overly sensitive and short tempered and will lash out for the smallest of reasons.  She gets a look in her face out of nowhere and we know trouble is seconds away.   She will throw everything off tables, spit, become aggressive towards anyone around her. 
• Extremely impatient
• Acts like a baby lots of times (jealous of sister???)
• Isn't fond of change.  Ex: Certain blankets, pillows, sheets, plates or bowls, etc...
• Does not follow rules despite being punished (time-out, privileges and possessions taken away)
• Sleep habits are all out of whack.  Goes to bed at 8-ish, wakes up around 11:30 pm (screams but just needs to be tucked back in), up again around 2 am (screams but just needs to be tucked in again- asks for bottle but doesn't get one), up again between 3-4:30 am requesting to watch television (I sometimes give in or else she screams and wakes up her sister), goes back to bed around 6 am and up for the day at 7:30 am  .... Naps for about 1 hr between 11 am and 1 pm

Reasons why I am not concerned:

• Makes great eye contact when she isn't pissed off or being defiant
• Great pretend skills
• Loves to play with strangers and make friends
• Can be very sweet when she feels like it
• Can undress herself and tries to dress herself but has some struggles.
• Feeds herself with all utensils
• No major sensory issues besides hating when she is dirty.

I am sharing this very personal info only because I am in desperate need of your advice, suggestions, comments, criticisms...anything you have to offer.  My heart aches for my baby girl.  I feel like she is suffering inside.  I sense that she is sad and that there is this other person inside of her trying to get out.  

Am I overreacting??