Ok it seems you read the documentation much more thoroughly than I ever did haha, so if the *reg* can be removed go for it, I didn't understand your point about this. You say it works and that's all that matters.
Yeah login takes a bit of work but its still ok, it does work and does let you do what you want. Basically Modx loses a bit of ease in getting started in exchange for significant customization capabilities. By comparison, Wordpress for example can get you up and running quickly, but you will start bumping your head on a very hard and unyielding ceiling very quickly, and there are no easy fixes there. I remember trying it and throwing a simple html template in there and it didn't come out looking right, and its hard to fix because somewhere in the bowels of the Wordpress engine its doing funky stuff that only some real experts understand.
I just mentioned the user group as I know you need to hook it all up to the forum. BTW, you can assign users to multiple groups at once if you want.
You have made a lot of quick progress! The learning curve is a little steep but it seems you have traversed it already, it gets more fun going forward!
By the way the *snippet* is the whole part with brackets around it [[ ]], and if I am not mistaken the exclamation point calls the snippet uncached, so its the updated snippet (I might have this a bit wrong, but that matters sometimes, if you had a snippet that got the current user, for instance, you would need to call it uncached for sure.) Snippets are basically tools to do stuff, injecting some dynamic code into a page, this is a major advantage of Modx. In my site I have a tool which grabs the most common words on the current page and searches the database for other pages with those words, so if someone was reading about glassware for instance the snippet would try to provide other pages with that keyword. Ideally this keeps the user clicking and clicking away while learning more and more.
Chunks, on the other hand, are pieces of more basic code like html, and the idea there is more like having a bit of code in one place, even if it appears in many places, so when you change it you only need to change it in one place. An example here would be, imagine I had 3 sites that all gave the same phone number and contact info, well I can put that content into a chunk and call it on all three sites, but when I change it there is just one location.
Any more questions just ask.