Personal Opinions On These Information Sources Is Appreciated Greatly

FYI - Most terms are listed in the CCF Glossary. A link is on the menu toward the top at the left of the screen.

These are both examples of hardness or microhardness tests used in metallurgy. Knowing the hardness of a metal can often provide clues about the prior heat treatment and current strength of an object. I would argue that this information is not that useful to numismatists and propose that you consider XRF for determining alloys.

All of the relevant US coin information is already available on CCF, at the "US Coin Facts" link in the left-hand column.

You should have Error-ref bookmarked and immediately available for just about any error research. It's the consummate resource for such research, authored by the best in the business.

NGC only attributes what they feel like attributing, and that's nowhere near the total of what's out there. Every issue has its' own "prime source" for error and variety information; Classic coins tend to have issue-specific sites and newer coins are covered by larger aggregate sites such as Variety Vista. Lincolns, specifically, have a few high-quality dedicated resources.