Do you have Google Webmaster Tools installed? Generally you will receive a warning from Google before your site is penalized, unless you are talking about just specific keywords. If you can pinpoint the exact dates of the drops, you should be able to see if they coincide with Penguin updates or not.

Yes, suffixes matter, tenses matter, singular vs plurals matter. These are all different keywords. Search for "build" vs "building" in Google, and you will get different results. Look up the search volume in the Adwords tool, and you will see that the keywords have different search volumes.

TLDR: Yes, it matters. Choose the version that has the highest search volume or your customers tend to use the most.

He's looking for a way to find which pages aren't indexed, not how many pages are indexed.

You can start by trying the "site:domain.com" search. This won't show you all the pages which are indexed, but it can help you determine which ones aren't indexed.

Another thing you can do is go into Google Analytics and see which of your pages have not received any organic visits. If a page has not received any clicks at all, there's a good chance it hasn't been indexed yet (or just isn't ranking well).

Finally, you can use the "site:domain.com/page.html" command to figure out whether a specific page is not being indexed. You can also do "site:domain.com/directory" to see whether any pages within a specific directory are being indexed.

Try:

RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^([^.]*[^./])$ /$1.html [R=301,L]

Try my code without all the other redirects, and see if it works. If it does, then add back the other redirects one by one until everything works.

You repeat this code a few times, maybe that's the problem? Pretty sure you only need it once:

RewriteEngine On
Options +FollowSymlinks
RewriteBase /

The line:

RewriteEngine On

Also only needs to be included once in an htaccess file. You may want to remove all the other instances.

Try adding this code at the very top, after the first "RewriteEngine On":

RewriteCond %{REQUEST_URI} ! .html$
RewriteCond %{REQUEST_URI} ! /$
RewriteRule ^(.*)$ $1.html

You are definitely missing out on image traffic by not hosting your own images. Plus, hotlinking is poor netiquette since you are using someone else's bandwidth without their permission. If the images are copyrighted, then you could be hit by DMCA requests which can negatively impact your SEO.

A canonical tag won't physically redirect you when you visit the page, it just lets the search engines know which is the right page to index.

If you want to actually redirect using .htaccess, try using this code

RewriteEngine On

RewriteCond %{REQUEST_URI} ! .html$

RewriteCond %{REQUEST_URI} ! /$

RewriteRule ^(.*)$ $1.html

Add a canonical tag to your header so that Google/Bing knows which version of your page they should be indexing.

You can also try looking into where the link to the non-html page is coming from. If it's an internal link, just change it so that Google doesn't continue to crawl it.