Hi Peter,
It's best to ask only the MediaWiki mailing list about this rather than cc'ing the Wikitech list which is meant for Wikimedia's own projects rather than external projects that use MediaWiki.
There's a FAQ at http://meta.wikimedia.org/wiki/MediaWiki_FAQ and MediaWiki forums at http://mwusers.com/ which you may find answer some of your questions.
1 - Sign in as admin. Is that so hard? How do I sign in as admin?
Go to [[Special:Userlogin]] on your wiki. You will have created an admin account when you installed MediaWiki. If you don't remember that user's name, try looking at [[Special:Listadmins]] on your wiki to see if there is anyone with admin access. If not, you'll need to add that via the database.
2 - Make others sign-in as lesser-editors and contributors. I need to see how hierarchies of user-capabilities work before I go live.
Other users should be able to create normal accounts at [[Special:Userlogin]].
3 - Lock parts of article contents so users can add to them but not delete the locked part, admins can change anything.
You can only lock an entire article, not part of it. A way around that is to add content to a template and lock the template. For example, put your content at [[Template:Main]] and type {{main}} on a page to include that content there.
4 - Change/customize a few visual things, like the sidebar.
The sidebar can be edited by any admin at [[MediaWiki:Sidebar]].
Angela