We'll look into the How To to ensure that it is correct.
Here are some responses to your other questions...
You can upload PowerCMS to any directory you desire, the cms100 directory is not necessary at all. As you've already experienced, you just need to enter the appropriate paths in the WA_Globals files for everything to work.
As far as WebAssist extensions and updates go, don't be surprised if there are some differences in the code you see in a Solution Pack vs. what is produced by an extension. For example, CSS Form Builder was developed after PowerCMS was launched and was therefore not used in the creation of the Solution Pack.
You should have no problems applying iRite to other pages on your website. The only issue would be if you are adding it to pages within the cms100 directory where you have PowerCMS stored, it may try to overwrite the WA_iRite directory and cause issues.
