mod_rewrite in Wordpress installation to replace multiple keys/variables

I am trying to create mod_rewrite rules to redirect the following url in my wordpress site:

www.mywebsite.com/pg/2/row/20/filter/all/filtercity/foo,bar

to

www.mywebsite.com/detail?pg=2&row=20&filter=all&filtercity=foo,bar

My problem is that wordpress included the following rules in the .htaccess:

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

# END WordPress

I tried the following RewriteRule and it kind of works if I comment the last RewrireRule from Wordpress.

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
#RewriteRule . /index.php [L]]
RewriteRule ^pg/([a-zA-Z0-9]+)/?$ ?pg=$2 [R]
</IfModule>

# END WordPress

The above works only for urls like

www.mywebsite.com/pg/2 => www.mywebsite.com?pg=2

A few questions:

1 - how can I configure .htaccess to process the url only when it finds the following keys

/pg and/or /row and/or /filter and/or /filtercity

for all other urls, it should execute the standard Wordpress RewriteRule . /index.php

2 - also the url can have all of the keys or only a few of them. For example:

/pg/2/row/20

/row/20/filter/all

how can I configure the .htaccess to process all the different combinations of keys?

3 - when I tried replacing

RewriteRule ^pg/([a-zA-Z0-9]+)/?$ ?pg=$2 [R]

with

RewriteRule ^pg/([a-zA-Z0-9]+)/?$ detail?pg=$1 [R]

I got a 404 error from the server. Not sure why. Any ideas?

Thank you.

EDIT 1:

I tried adding [L] to the end of my RewriteRule (see below)

RewriteRule ^pg/([a-zA-Z0-9]+)/?$ ?pg=$1 [R,L]

and moving the standard Wordpress rewrite rule to the next line

RewriteRule . /index.php [L]

After doing that I no longer get server 404 error. However the CSS files stop loading.