It is important to note that quota settings, and tests fro compliance, only happen once per day. This is important becuase if you set up a quota for a user/account today, it will not be enforced until sometime after the next quota processing routine is run - in the meantime, you will still be runnng with the previous, if any, quota. It also mean that a quota will not be determinied to have been exceeded until after it has happened and the next quota processing routine is run.
This is why DreamHost warns that you should set quotas (particularly in the bandwidth area) with some "buffer" so that it will be enforced a closely as possible to what you intend.
For example, if I may want an user to only use a maximun of 50MB pf disk storage, I might set his quota to 25-30 MB in hopes of him never breaking 50MB before the quota is evaluated.
As for your other questions:
See previous discussion above. Yes it does have a value, but it is subject to the way the quota system works.
It's not likely to impact the speed of the webserver (email is a different machine than the web server)
Bandwidth usage is determined by user not domain or website
No. Quotas are updated daily, so you won't see accurate statistics until the following day.
No, but note that only full machine users have such directories and ftp space . Email only users do not, and you cannot directly manipulate their Maildirs.
I'm sorry, but I do not understand that question - you can track used ("busy"?) email space by user, or by domain
Not that I am aware of.
I don't believe email traffic (bandwidth) is a concern (I don't know that it is even metered) but you should worry about disk storage space if you think there is any chance that you might exceed your account quota.
I'm not sure what you are asking here, but I do not see any relationship between the bandwidth and disk storage usage and the "speed" of web server processing.