I would say that it is worth the money if you have a video that is more than a couple minutes. They charge $1/minute of video. They give you the file back within a day or two.
Alt text is that HTML code that sits on top of an image, link, style, media player, etc. Moodle encourages/prompts users to include it when they add that type of content for the purposes of accessibility. It’s what screenreaders and other accessibility tools identify or ignore (for instance, if you have an image that is decorative, you don’t need to identify it and you want screenreaders to skip it so it gets what’s called a null tag alt=""
And yes, it would read the descriptive text to the users.
We do, before we recently switched over to an Instructional Design method of PD Development, we made this PD Writing Course. I will email you the enrollment key if you want to check it out. There is a section there about accessibility. If any other 287 employee wants the key let me know. Also, if you’re interested in doing more Instructional Design work, you’ll need to view all this stuff