1) The only thing it's missing is a book that was already a PWYW book. It has a different team than the normal book, but
you can get that here.
2) "Best way" to read is largely subjective. You'd get multiple digital file formats through humble - generally .pdf, .epub, and .cbz, at least one of which should work on whatever devices you have available. Larger tablets are generally the preferred way to read comics digitally but I'll often read them on a desktop and a lot of people swear by Kindles.
3) Yes, but you'd be paying a lot more and some of the specials/spin-offs may not be in them. The Walking Dead has "deluxe"
omnibuses with about 24 issues in them (roughly 4 trades) and
compendiums with 48 issues. I generally don't find books that large particularly pleasant to read, but compendiums are the best bang for your buck in physicial form. You'd then need to pick up things like
Here's Negan separately...and possibly track down the Free Comic Book Day special if you want everything.
4) I only got about 2 seasons into the TV show before given up. While the shows are certainly thematically similar, plot points differ pretty greatly.
You could try buying the $1 tier - which still gets you a lot of comics - to see if it keeps your interest. Humble used to let you increase your payment a week after something closes and still allowed you to get the extras, but even if they still don't do that it would allow you to determine if you want to keep reading (and most libraries have Walking Dead comics).