In some ways you are expecting more than would be reasonable for a child so young, particularly in the area of speech development and use of language. She is very young, too young to expect any organized type of learning, such as the alphabet. Overall she seems to be doing well relative to her emotional development and social engagement. Now, there may be some concerns in her gross motor development and language development, and these do invite evaluation. The best approach would be to arrange a developmental evaluation, which would take a look at the broad scope of her development (gross and fine motor, cognitive, emotional, social). A second approach would be to refer her to Early Intervention (this would be a simpler arrangement).
By the way, at 18 months she is too young to learn ABCs and such. When teaching her to use language, you need to make it fun and not annoy her. Try to encourage her to speak when she is interested in something or wants something. Let's say that she really wants her juice cup. You need to tell her "you want the juice cup". She probably will say "cuppp". You reply back: "yes, cup... good job princess".
Have you mentioned to her pediatrition your concerns? Did he/she have her hearing tested? Either way, my older daughter was like yours. As a mom, if you suspect something, you need to act on it. You probably should have a full evaluation done by a child development doctor and/or your state's Children Early Intervation program (including hearing and speech pathology tests). It seems that her speech is a little delayed and the earlier you start helping her with professional speech therapy the better her chances of progressing and catching up. My daughter was like yours and started speech therapy at 20 months. It turns out that she has ADHD. She is now in 1st grade and doing well. Given how affectionate she is and follows commands, I do not think that yours has Autism or ADHD. Do listen to your instincts and concerns and obtain professional help. There is so much help out there for speech delay.