Arch can definitely be used by beginners. The only required skill is the willingness and ability to invest time into the process. There shouldn’t be a minimum knowledge bar to try Arch. However, the willingness to RTFM and Google are undoubtedly necessary.
You have some good ideas about useful things to know during first time installation. However, the installation guide is not the place for this. The current format is a good compromise, and it is not the right place to provide more background. Instead, supplementary information should be added to existing pages/new pages and linked from the guide.
I think the problem with the current guide is it links out to too much context. It should only link to pages that are absolutely necessary to be read and any further diving should happen from the linked pages. Note: I don’t think the actual guide should have more information added, but rather that some information should be moved.
The forest-of-links approach is the one that scales and doesn’t get a bunch of outdated documentation. A beginner can and should read some of the linked pages. This is part of the time investment.
The install script is fairly new. It’s also not aimed at new users, but rather experienced ones looking to automate some of the process. The support issue you ran into is a perfect example of why forest-of-links is the better approach. The support forum thread has become out-of-date, and is now providing inconsistent information by saying that manual installation guide installation is the ONLY supported method.