Would tin (II) sulfide be considered a covalent network solid?
There is no sharp border between ionic and covalent compounds, much like there is no sharp border between a boy and a man. Some compounds between a metal and a non-metal, like $\ce{LiF}$ or $\ce{NaCl}$, are predominantly ionic, some (think of $\ce{Mn2O7}$ or $\ce{TiCl4}$) have markedly covalent character, and the rest are somewhere in between. As you see yourself, $\ce{SnS}$ is ionic enough to have certain kinds of reactions typical of ionic compounds, but not so ionic in terms of electronegativity or crystal structure.

Now, people in some areas of chemistry would put forth some definitions to tell covalent from ionic. Your "difference of electronegativities of at least 1.7" is one of those. Another is based on $\ce{MgO}$ and would use it as a border. There are a lot of definitions, and it's not like they are all wrong or useless; in fact, they are necessary to keep our stuff in order. One has to remember, though, that they are all slightly different, and all inherently arbitrary (much like the legal age limitations for driving, voting, drinking, and other activities, if we'd talk once more in terms of the "boy or man" analogy).

This must have been said here like a dozen times before, but whatever. Repetition is the mother of learning, they say.