While toddlers will get flagged for not having five words at 18 months or 50 at 24 months to make sure something isn't wrong, it is still within the range of normal for a child not to say his first word until he turns two. I believe the average at 24 months old is somewhere between 150 and 300 words. And, approximations still count a this age.
I'm not saying your DD is not language delayed. I don't know. But, I think it is easy to expect more from your child than what is fair especially when they are showing uneven development.
Also, 18 months was the peak of our DD's stranger anxiety, and that did not get better until right around 2.5. What you are describing sounds pretty normal to me. Our DD would try to claw her way inside me around her grandparents at that age. She was fine out in public until anyone got in her face.
The obsession with letters does not seem unusual for this age to me, either. As long as it is not interfering with her daily life I think it is okay. It does seem to be an age for obsessions, whether trains, dinosaurs, construction vehicles, letters, or backyardiagans. The fact that she is into pretend play bodes well.
