Learn English – devo — what does that word mean

Gershenfeld uses the phrase "play that forward" a couple of times. The metaphor comes from playing a tape forward. He's moving backwards and forwards in a history of computation and digital fabrication, so what he means is, let's see how these things developed. Let's continue this history from this point, or with these factors in mind, and do so in a chronologically forward direction.

what with relatives and everything

could also be written as:

what with relatives and all

It means: Considering relatives and others that they know, each person being acknowledged are expected to result in sales of more than one book.
This is a common view when low sales are anticipated. Direct sales might be low, but they may talk others into buying.