Multisite: one database, some shared and some unique content

I am trying to create a multi-site community where each site gets much of its content from the main site but some of the content will be unique to the site. I have searched the forums and google and haven't been able to resolve some of the issues.

My goal is to have a group of sites with a common core interest but serving different communities. For example, the main site (www.mainsite.com) would have the common stuff (such as news posts and blogs about a geographic region--like Colorado, USA). Each community site would contain everything from the main site but also have information about the specific community.
--A farming community site would have info on farming in Colorado plus the common content from the main site.
--A recreation community site would have info on recreation in Colorado plus the common content from the main site.
--An education community site would have info on education in Colorado plus the common content from the main site.
The community specific info would not be visible on the other community specific sites. A central administrator adds content to the main site. Authenticated users add content to the community sites. Users can log into any of the community sites (i.e. shared user tables). Some of the unique content on the community sites will be specific to groups (i.e. a sports league) so I will be using organic groups.

I currently have multiple sites (using subdomains currently) working with the different themes for each and all content is shared (though I can manage the blocks to hide some content from non-savvy users).

Here is the setup using the example above:
Drupal installation is in www.mainsite.com/drupal

the /sites directory contains:
/default
/themes
/defaultheme
settings.php
/farming.mainsite.com
/files
/themes
/farmingtheme
settings.php
/recreation.mainsite.com
/files
/themes
/recreationtheme
settings.php
/education.mainsite.com
/files
/themes
/educationtheme
settings.php

I currently am sharing the entire database with no prefixed tables.

each settings.php has:
$db_url = 'mysql://username:password@localhost/databasename'; (common in each site)
$base_url as http://farming.mainsite.com (unique to each site)
$conf = array(
'site_name' => 'Farming Community', (unique to each site)
'theme_default' => 'farmingtheme' (unique to each site)
);

I have 3 current problems/challenges:
1) The community sites currently cannot see files/images uploaded from the main site. These are currently in /drupal/files under the "Public Download" method. We expect this site to become quite large so we don't want the overhead of the "Private Download" method. The node_revisions table shows the img src for the main site files as . My guess is that a directory "relative to the drupal installation" as required under "Public Download" is different for the main site vs. the multiple sites. Any ideas?

2) How can I make the content added by users in each community site visible only to that site? If I use taxonomy it seems that a user entering a blog on farming.mainsite.com would have to make sure he/she chose the "farming" category and not "recreation" or "education" since all three would be visible on each site, right?

3) I am having problems with authenticated users trying to log in to the community sites. As an administrator I can see that a session was opened for them when they enter their user name and password, but they never "enter the site" as the User Login block remains and they cannot function as an authenticated user--just as an anonymous user. This is not the same problem as the occasional "login twice" problem as these users never get in.

I admit I am a relative newbie at this. But from reading the forums, etc. it appears that this functionality is possible but I haven't found solutions or resolutions.

Thank you so much in advance for any help you can provide!