Yes, you can configure it so each domain or sub-domain is under the control of a different machine user. Logging-in via FTP as a particular user will show only the domain files under the control of that user.
A lot depends on how Simplehost has things configured, but you may be able to use something like wget in the Shell to transfer the files directly between the servers.
Not being a big Gallery user, I can't really answer this question, but, as you say, having the one-click-install and the unlimited MySQL databases may make this less important anyway.
Yes, you can create a temporary (free) dreamhosters.com sub-domain and set it to mirror your real domain. The procedure is outlined in the wiki article below;
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