No. There are a number of edge cases where the expiry date is incorrect - the USPTO even provides a patent expiry calculator. For patents issued 20 years ago, "issue date + 20 years" should hold true as a very basic rule of thumb. Please do not build a business based on one of the displayed patents without first checking what other, newer patents may also affect said invention - the site is merely meant as a starting point to get inspired; the actual research should follow suit.

Generally, yes. The underlying idea of the patent system is is that for a given number of years, your invention is protected, and can only be used by others with your consent. After it has expired, you can't re-patent it "as is", but you can sometimes patent a derivative (e.g. an improvement of the same base idea). A common issue is that there sometimes can be a number of patents behind an idea, often interlinked, so it is frequently difficult to determine whether you can "use" the expired patent.

I believe so. The majority of patents are never commercialized by their original inventors - yet, they went through the (often costly and time-consuming) process of patenting it. This in itself obviously doesn't guarantee that there is a viable target market for this idea, or that it can be produced at a reasonable price-point. You should use this site as a trove of inspiration, not as a collection of blueprints.

This depends heavily on the product itself and the industry. For a preliminary read, I recommend this book by Dan Olsen (affiliate link).

Simply shoot me an email at nick@patentsexpiringtoday.com!