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You will also have to replace your database (due to running upgrade.php from 1.5.2 >> 2.0). The easiest way is to use the last db backup taken before you upgraded. Alternativly is the database downgrader. Make sure you backup your 2.0 database and files first incase anything goes wrong.

Just out of interest, what is it you don’t like about 2.0? There are a couple of plugins that can somewhat restore the 1.5x appearance/functionality:
Admin revert” and
Old style upload
You can loose the uploader via changing the wp-admin.css file:
Change:
#uploading {
border-style: none;
padding: 0px;
margin-bottom: 16px;
height: 15em;
width: 100%;
/* overflow-y: hidden;*/
}
to
#uploading {
display: none;
}

The main protagonists are linked on this page, and I would think you should be able to find contact details on their blogs.

Guessing you are using SK2 (after snooping in your plugins folder)… You need to update to the latest version SK2.1x and get the compatability plugin.
http://wordpress.org/support/topic/53323

A simple question: No

As far as is known 1.5.2 is secure.

Depending on how frequently your ‘live’ updates are, you could install wp-cache and set the cache-timer to one minute (or longer or shorter as appropriate) this would result in mostly serving hmtl rather than hammering the server with so many database queries.

http://mnm.uib.es/gallir/wp-cache-2/

It will be in your theme footer – something like /wp-content/themes/themeyouareusing/footer.php. Or you may be able to edit the footer through the presentation menu.

I am not sure. I think there are some security issues with 1.5.1 so it may be wise to upgrade to 1.5.2 anyway. (I think there is a post somewhere that batch categories does not work with 2.0).

http://static.wordpress.org/archive/ for older versions.

If you are using 1.5.2 you can use Batch categories