Does using a long tail keyword as an image alt text be counted in keyword density?

Yes, but keyword density should not be an issue today unless you are doing a ton of keyword stuffing. Anything you write should be natural for the website visitor. You can include semantic phrases that relate to the phrase that you are trying to rank for. In fact, I have seen evidence of a page ranking for a certain keyword phrase when that particular keyword phrase was not even mentioned once on the page!

For alt tags, anything you put in there has to describe the image itself. Anything else would be considered spam. Alt tags are used for screen readers for the blind so that the blind know that the image is about. If you have an image with text in it then put that text in the alt tag. If you have an image of a pie chart then describe what the pie chart represents.

You can use the HTML code ‘desc’ to add a longer text version of what the image is about that will help you with what you want to do. WordPress makes this easy by showing you a box to add text for a description for every image you upload to it. I have found that using the description box in WordPress really helped with product rankings.

How do you track mentions about your company and relevant keywords on forums and blog posts, etc.? Are there any free tools to use?

Then you can use URL tracking codes to track links and view them in Google Analytics which are both free.

You can use advanced Google Search Operators to view relevant keywords in forums. An example would be like this one.

cars site:someforum.com

This typed into the search engine above will show you all of the pages that mention cars in only the website someforum.com.

How can I start learning digital marketing online? There are so many courses, so many different topics (SEO, social media, Google ads, etc.), and I get lost. How can I get organized?

I was in the same boat about 16 to 17 years ago. It took a while but I began to learn with sources that were trustworthy. As of today, I would trust Moz’s Whiteboard Fridays, videos, and articles, SEMRush articles, Ahrefs, Search Engine Journal, Search Engine Land, Aaron Wall, etc.

How can I write SEO-based content?

Find what hot topics are searched for the most with the least competition via keyword research, Google Trends, Social Media, and write about them using infographics, top 10 lists, in-depth professional, authoritative content.

It would be a bonus if you can answer a question online that has not been answered yet. You can rank pretty quickly with that content.