1. If humans are at the top of the food chain, is our DNA as complicated as it gets? Are there any plants or animals with more complicated DNA?
We are not at the top of the food chain; insects and bacteria are. They eat us.
I am pretty sure (but don't know) that by some measures of complexity (number of genes, chromosomes, base pairs) we are not the most complex. It may well be that by other measure we are but I don't think that is known yet.
2. Does human DNA contain the sum total of all the DNA that has gone before us. In other words, can human DNA be "read" as the greatest Natural Biology history text of Earthly fauna there is?
Yes and no. The totality of the gene pool of all organisms is a record of all that has gone before and the enviroments they adapted to. However, the record has been written over and written over again and as you go back it is more jumbled like a palpimset (sp?) that has been used more than once.