Force HTTPS for every pages except one - .htaccess

Question

I want to force HTTP for one page i.e. preview.php. Rest the website can use HTTPS. Because preview.php has an iframe which loads the content from NON-Secured locations. And hence do not display on a secured page.
I have tried several solutions here on Stack Overflow but didn't work. Any suggestion will be appreciated. My code as follows:
If there is any mistake in code below. Please me know.:
I have tried the following solutions mentioned here :
Force HTTPS on certain URLs and force HTTP for all others
Force https for some pages and http for all others
How to force rewrite to HTTPS except for a few pages in Apache?
Using .htaccess to control HTTPS on certain pages
<FilesMatch "(?i)^.*\.(ico|flv|jpg|jpeg|png|gif|js|css)$">
ExpiresActive On
ExpiresDefault A432000
</FilesMatch>
Options +FollowSymLinks -MultiViews
# Turn mod_rewrite on
RewriteEngine On
RewriteBase /
SetEnvIfNoCase User-Agent "^libwww-perl*" block_bad_bots
Deny from env=block_bad_bots
# index.php to /
RewriteCond %{THE_REQUEST} ^GET\ /.*/index\.(php|html)\ HTTP
RewriteRule (.*)index\.(php|html)$ /$1 [R=301,L]
# force www.
RewriteCond %{HTTP_HOST} ^www\.domain\.co [NC]
RewriteRule ^(.*)$ http://domain.co/$1 [L,R=301]
RewriteCond %{HTTPS} ^off$
RewriteCond %{REQUEST_URI} !preview\.php$
RewriteRule ^.*$ https://%{HTTP_HOST}%{REQUEST_URI} [R,L]
#RewriteCond %{HTTPS} off
#RewriteCond %{REQUEST_URI} !/preview
#RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [L,NE,R=301]
# To externally redirect /dir/foo.php to /dir/foo
RewriteCond %{THE_REQUEST} ^[A-Z]{3,}\s([^.]+)\.php [NC]
RewriteRule ^ %1 [R=301,NE,L]
RewriteRule ^profile/(.+)$ profile.php?username=$1 [L,NC,QSA]
RewriteRule ^dashboard/(.+)$ dashboard.php?option=$1 [L,NC,QSA]
RewriteRule ^theme/([a-zA-Z0-9-/]+).html$ theme.php?themename=$1
RewriteRule ^theme/([a-zA-Z0-9-/]+).html/$ theme.php?themename=$1
RewriteRule ^preview/([a-zA-Z0-9-/]+).html$ preview.php?themename=$1
RewriteRule ^preview/([a-zA-Z0-9-/]+).html/$ preview.php?themename=$1
RewriteRule ^category/([0-9]+)$ category.php?page=$1
RewriteRule ^category/([0-9]+)/$ category.php?page=$1
RewriteRule ^category/([a-zA-Z0-9_-]+)$ category.php?cat=$1
RewriteRule ^category/([a-zA-Z0-9_-]+)/$ category.php?cat=$1
RewriteRule ^category/([a-zA-Z0-9_-]+)/([0-9]+)$ category.php?cat=$1&page=$2
RewriteRule ^category/([a-zA-Z0-9_-]+)/([0-9]+)/$ category.php?cat=$1&page=$2
RewriteRule ^category/([a-zA-Z0-9_-]+)/([a-zA-Z0-9_-]+)/([0-9]+)$ category.php?cat=$1&subcat=$2&page=$3
RewriteRule ^category/([a-zA-Z0-9_-]+)/([a-zA-Z0-9_-]+)/([0-9]+)/$ category.php?cat=$1&subcat=$2&page=$3
RewriteRule ^category/([a-zA-Z0-9_-]+)/([a-zA-Z0-9_-]+)$ category.php?cat=$1&subcat=$2
RewriteRule ^category/([a-zA-Z0-9_-]+)/([a-zA-Z0-9_-]+)/$ category.php?cat=$1&subcat=$2
RewriteRule ^header.php page-not-found.php [R=301,L]
RewriteRule ^footer.php page-not-found.php [R=301,L]
# To internally forward /dir/foo to /dir/foo.php
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME}.php -f
RewriteRule ^(.+?)/?$ $1.php [L]
<ifModule mod_gzip.c>
mod_gzip_on Yes
mod_gzip_dechunk Yes
mod_gzip_item_include file .(html?|txt|css|js|php|pl)$
mod_gzip_item_include handler ^cgi-script$
mod_gzip_item_include mime ^text/.*
mod_gzip_item_include mime ^application/x-javascript.*
mod_gzip_item_exclude mime ^image/.*
mod_gzip_item_exclude rspheader ^Content-Encoding:.*gzip.*
</ifModule>
<IfModule mod_deflate.c>
# Compress HTML, CSS, JavaScript, Text, XML and fonts
AddOutputFilterByType DEFLATE application/javascript
AddOutputFilterByType DEFLATE application/rss+xml
AddOutputFilterByType DEFLATE application/vnd.ms-fontobject
AddOutputFilterByType DEFLATE application/x-font
AddOutputFilterByType DEFLATE application/x-font-opentype
AddOutputFilterByType DEFLATE application/x-font-otf
AddOutputFilterByType DEFLATE application/x-font-truetype
AddOutputFilterByType DEFLATE application/x-font-ttf
AddOutputFilterByType DEFLATE application/x-javascript
AddOutputFilterByType DEFLATE application/xhtml+xml
AddOutputFilterByType DEFLATE application/xml
AddOutputFilterByType DEFLATE font/opentype
AddOutputFilterByType DEFLATE font/otf
AddOutputFilterByType DEFLATE font/ttf
AddOutputFilterByType DEFLATE image/svg+xml
AddOutputFilterByType DEFLATE image/x-icon
AddOutputFilterByType DEFLATE text/css
AddOutputFilterByType DEFLATE text/html
AddOutputFilterByType DEFLATE text/javascript
AddOutputFilterByType DEFLATE text/plain
AddOutputFilterByType DEFLATE text/xml
# # Remove browser bugs (only needed for really old browsers)
BrowserMatch ^Mozilla/4 gzip-only-text/html
BrowserMatch ^Mozilla/4\.0[678] no-gzip
BrowserMatch \bMSIE !no-gzip !gzip-only-text/html
Header append Vary User-Agent
</IfModule>
ErrorDocument 404 /page-not-found.php