Solution:
I recommend to use two .htaccess files: one in your wordpress directory which catches all requests which should be handled by WordPress and one in your root directory for the rest.
I’m not so sure about the %{QUERY_STRING} in your rewrite rules … test it thoroughly. Also, I don’t have any experience with Magento. The last time I tested it, it was way too slow. 🙂
As a start, here are the two .htaccess files:
/wordpress/.htaccess
RewriteEngine On
RewriteBase /wordpress/
# WordPress allows URLs like /2010/0/ or /00/ == doubled content
RewriteRule (^0+|(.*)/0+)/$ /$2 [L,R=301]
# WordPress: paged content creates a /page/
RewriteRule (^p|(.*)/p)age/(1/)?$ /$2 [L,R=301]
# Images, Stylesheets etc.
RewriteCond %{REQUEST_URI} !.+\.\w{2,4}$
# Existing file
RewriteCond %{REQUEST_FILENAME} !-f
# Existing directory
RewriteCond %{REQUEST_FILENAME} !-d
# Symbolic link
RewriteCond %{REQUEST_FILENAME} !-l
RewriteRule ^ index.php [L]
root /.htaccess
RewriteEngine On
RewriteBase /
RewriteRule ^(.*)-p-([0-9]+).html$ product_info.php?products_id=$2&%{QUERY_STRING} [L]
RewriteRule ^(.*)-c-([0-9]+).html$ index.php?cPath=$2&%{QUERY_STRING} [L]
RewriteRule ^(.*)-m-([0-9]+).html$ index.php?manufacturers_id=$2&%{QUERY_STRING} [L]
RewriteRule ^(.*)-pi-([0-9]+).html$ popup_image.php?pID=$2&%{QUERY_STRING} [L]
RewriteRule ^(.*)-pr-([0-9]+).html$ product_reviews.php?products_id=$2&%{QUERY_STRING} [L]
RewriteRule ^(.*)-pri-([0-9]+).html$ product_reviews_info.php?products_id=$2&%{QUERY_STRING} [L]
RewriteRule ^(.*)-t-([0-9_]+).html$ articles.php?tPath=$2&%{QUERY_STRING} [L]
RewriteRule ^(.*)-au-([0-9]+).html$ articles.php?authors_id=$2&%{QUERY_STRING} [L]
RewriteRule ^(.*)-a-([0-9]+).html$ article_info.php?articles_id=$2&%{QUERY_STRING} [L]
# Information pages
RewriteRule ^(.*)-i-([0-9]+).html$ information.php?info_id=$2&%{QUERY_STRING} [L]
RewriteRule ^(.*)-links-([0-9_]+).html$ links.php?lPath=$2&%{QUERY_STRING} [L]
RewriteRule ^(.*)-n-([0-9]+).html$ newsdesk_info.php?newsdesk_id=$2&%{QUERY_STRING} [L]
RewriteRule ^(.*)-nc-([0-9]+).html$ newsdesk_index.php?newsPath=$2&%{QUERY_STRING} [L]
RewriteRule ^(.*)-nri-([0-9]+).html$ newsdesk_reviews_info.php?newsdesk_id=$2&%{QUERY_STRING} [L]
RewriteRule ^(.*)-nra-([0-9]+).html$ newsdesk_reviews_article.php?newsdesk_id=$2&%{QUERY_STRING} [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-l
RewriteRule (.*) /index.php?cPath=$1 [L]
Your web hoster has a very informative RewriteLog, which may help if you still have any problems.