Well, the htdocs/_lib/_base is a relative path, not an absolute one... that means that the path is interpreted differently depending on where the .htaccess file is located.
Your best bet is to always use absolute paths when specifying your include paths. You can always double check your settings using phpinfo().
When you configure PHP using php.ini or httpd.conf, your settings become the "master value". When you override those changes in .htaccess files, those become the "local value".
The last thing you need to do is configure Apache to allow configuration changes in .htaccess files. You need to enable the Options override for the directories you want to modify php config changes in.
Search the Apache manual for the AllowOverride directive. You need to have "AllowOverride Options" set for your directory.