I'd be glad to work with you. Screen share via Google Hangouts? Send me a message via http://glerner.com/contact.php I always receive those.
"That is the way of finding out what works, what doesn't, gathering leads, etc in all technologies."
Yes, and that is how I got my multi-site working... Grrr... So I wrote my documentation so knew I understood what was going on and could replicate it for all my client's sites.
"As far as I can understand it, you simply start with 2 and keep incrementing from there."
No, you have to find the ID of the site you want to map, and you put That in the field. Read my instructions on how to do that, or how to do that via the mapped site's Settings. [That's what I tried, wondering "what do I put in this field", so good guess. I somehow pieced together the hints from several documents... and of course it makes sense once I figured it out.]
"changing things but those changes never staying"
The plugin is not documented well enough. That setting for Site ID doesn't get edited that way; all you can change is the Mapping for that Site ID. Again, delete the bad record, enter a new one. [I know, I tried that too.]
"What you are telling people to do is to set up a wildcard domain DNS record such as *.domain.com. That's fine if all you'll be doing is setting up sub-domains, meaning 1.domain.com, 2.domain.com, 3.domain.com. There is no need to do this with your DNS server if you are not doing to be using sub-domains but will be using multiple domains."
Yes, and No, I mean you have to use Wildcard DNS if you are not going to manually set up DNS for each new site. With Wildcard DNS, WordPress will handle all the "virtual subdomain" creation; much less work for you as the Administrator. (For people on shared hosting, each DNS change has to be done by their hosting company, could take days; wildcard DNS is only once -- if your CPanel (or equivalent) won't let you set up wildcard DNS, make them figure out how to do it!)
Explaining for everyone: I suspect that for Domain Mapping to work, the unmapped domain has to work. If you are mapping salmon.fishing.com to my-salmon-fishing.com then fishing.com would have to be where WordPress is installed, and salmon.fishing.com has to display properly; then you can map the domain. Similar testing for subfolder setup (fishing.com/salmon).
One thing that Will Not work is having salmon as an Add-On domain (the way CPanel creates them), which would send requests to the folder on your account instead of to the WordPress index.php (public_html/index.php is correct, not public_html/salmon/index.php). 404 File Not Found errors is a good clue you got it wrong, check what specific file name it isn't finding.