RewriteRule one single node to another domain - .htaccess

Question

I want to use one node in Drupal 7 as a landingpage with its own domain.
Both domains are linked to the same folder, showing the same content.
www.domainA.com/landingpage should be www.domainB.com - nothing else ...
RewriteCond %{HTTP_HOST} ^(www.)?domainA.com$
RewriteRule ^landingpage http://www.domainB.com [R=301,L]
And i want domainB.com to show the content of domainB.com/landingpage:
RewriteCond %{HTTP_HOST} ^(www.)?domainB.com$
RewriteRule ^$ /landingpage [P]
This works.
Now I need to redirect all other pages from domainB back to domainA to avoid Duplicate Content:
www.domainB.com/allotherpages should be www.domainA.com/allotherpages
RewriteCond %{ENV:REDIRECT_STATUS} ^$
RewriteCond %{REQUEST_URI} !^/landingpage$ [NC]
RewriteCond %{HTTP_HOST} ^(www\.)?domainB\.com$
RewriteRule ^ http://www.domainA.com [R=301,L]
Alltogether it is (part of htaccess-file of Drupal 7):
<IfModule mod_rewrite.c>
RewriteEngine on
RewriteRule ^ - [E=protossl]
RewriteCond %{HTTPS} on
RewriteRule ^ - [E=protossl:s]
RewriteRule ^ - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]
RewriteRule "(^|/)\." - [F]
RewriteBase /
# HERE STARTS MY CUSTOM RULE-SET:
# Rewrite one node to new Domain:
RewriteCond %{HTTP_HOST} ^(www.)?domainA.com$
RewriteRule ^landingpage http://www.domainB.com [R=301,L]
# Front page of domainB shows content of one node:
RewriteCond %{HTTP_HOST} ^(www.)?domainB.com$
RewriteRule ^$ /landingpage [P]
# Rewrite all other pages from domainB.com to main domainA.com:
RewriteCond %{ENV:REDIRECT_STATUS} ^$
RewriteCond %{REQUEST_URI} !^/landingpage$ [NC]
RewriteCond %{HTTP_HOST} ^(www\.)?domainB\.com$
RewriteRule ^ http://www.domainA.com [R=301,L]
# HERE ENDS MY CUSTOM RULE-SET
<IfModule mod_headers.c>
# Serve gzip compressed CSS files if they exist and the client accepts gzip.
RewriteCond %{HTTP:Accept-encoding} gzip
RewriteCond %{REQUEST_FILENAME}\.gz -s
RewriteRule ^(.*)\.css $1\.css\.gz [QSA]
# Serve gzip compressed JS files if they exist and the client accepts gzip.
RewriteCond %{HTTP:Accept-encoding} gzip
RewriteCond %{REQUEST_FILENAME}\.gz -s
RewriteRule ^(.*)\.js $1\.js\.gz [QSA]
# Serve correct content types, and prevent mod_deflate double gzip.
RewriteRule \.css\.gz$ - [T=text/css,E=no-gzip:1]
RewriteRule \.js\.gz$ - [T=text/javascript,E=no-gzip:1]
<FilesMatch "(\.js\.gz|\.css\.gz)$">
# Serve correct encoding type.
Header set Content-Encoding gzip
# Force proxies to cache gzipped & non-gzipped css/js files separately.
Header append Vary Accept-Encoding
</FilesMatch>
</IfModule>
</IfModule>
"Works a little bit" – but breaks the layout and everything – i think, because it rewrites everything else (CSS-files), too ...
Can't find a solution for this.
Edit:
This seems to work – thanks to Jon Lin!
RewriteCond %{HTTP_HOST} ^(www.)?domainA\.com$
RewriteRule ^landingpage http://www.domainB.com/ [R=301,L]
RewriteCond %{HTTP_HOST} ^(www.)?www.domainB\.de$
RewriteRule ^$ /landingpage [P]
RewriteCond %{ENV:REDIRECT_STATUS} ^$
RewriteCond %{REQUEST_URI} !\.(css|js|png|svg|ico|jpg)$ [NC]
RewriteCond %{REQUEST_URI} !^/$
RewriteCond %{REQUEST_URI} !^/landingpage$ [NC]
RewriteCond %{HTTP_HOST} ^(www\.)?www.domainB\.de$
RewriteRule ^(.*)$ http://www.domainA.com/$1 [R=301,L]

Answer 1

First, I don't think you need the P flag. Since both domains point to the same place, you don't need to proxy the request:
RewriteCond %{HTTP_HOST} ^(www.)?domainB.com$
RewriteRule ^$ /landingpage [L]
For everything else, you need to capture the request and include that as part of the redirect:
RewriteCond %{REQUEST_URI} !^/$
RewriteCond %{REQUEST_URI} !^/landingpage$ [NC]
RewriteCond %{HTTP_HOST} ^(www\.)?domainB\.com$
RewriteRule ^(.*)$ http://www.domainA.com/$1 [R=301,L]
Before, you were redirecting everything to the landing page of www.domainA.com
If you want to outright prevent css and scripts from being redirected, you can add additional conditions to exclude them:
RewriteCond %{REQUEST_URI} !\.(css|js)$ [NC]
RewriteCond %{REQUEST_URI} !^/$
RewriteCond %{REQUEST_URI} !^/landingpage$ [NC]
RewriteCond %{HTTP_HOST} ^(www\.)?domainB\.com$
RewriteRule ^(.*)$ http://www.domainA.com/$1 [R=301,L]