I've written web  service methods that hit Active Directory, and are called
by InfoPath forms; that's probably the method I'd recommend.

The second part  is a little confusing. Do you really want all your AD info
in an actual SharePoint list, or do you just want to display a set number of
groups and users? If the latter, you could simply iterate through the
groups you want, making an LDAP call to get the users for each one; of
course, if the AD users will be SharePoint users, you could get the
information that way.

I'm just finishing a project that has a lot of these characteristics. It uses a
workflow built in Visual Studio with InfoPath task forms.

For the part  about displaying  SharePoint users and groups, you could use
this web  service: UserGroup.asmx