I can provide an interesting but non-rigorous perspective on whether or not you can teach a toddler to read.

My kids went to a small in-home daycare before they started school (12-15 kids). The woman who ran it is very bright and she used to do "school" with the kids every morning. One of the things they did every day was to learn letters and letter sounds (in addition to singing and a ton of arts and crafts). They also sounded out words.

Participation was voluntary. Kids could play if they didn't want to go to school. Most of them wanted to go to school because it was so much fun. They usually started when they turned 2.

I used to talk to this woman about how much the kids learned after 3-ish years of school with her. She told me that most of them would go into kindergarten knowing their letters and letter sounds, as well as being able to recognize a few sight words or even sound out some very basic words. She told me once that none of them learned to read like my DD had. Okay, DD wanted to work on reading at home, but at least some of the other parents had to have been doing stuff with their kids, too.

So, this was not a scientifically rigorous study by any means, but it was (continues to be) a nurturing environment that can't be called hothousing in the negative sense of that word. The kids were a fairly standard cross-section of the population: the parents were office workers from admins to engineers, as well as military people, people in allied health, tradesmen, etc. etc. I knew a very few kids who ended up in special ed. and a very few kids who will probably be in GATE programs (they're still too young).

Based on what I saw at daycare, I suspect that you can't teach a child to truly read before s/he's developmentally ready. There is presumably some sort of brain development that has to occur.