[Plugin: Redirection] Redirecting to WordPress 'Page' using passthrough

I am having a confusing problem.

I created a page with a URL shaped like this:
<domain>/Page_name/?name=<personal name>

Where ‘Page_name’ is the name of a page I configured in WordPress, that contains PHP code executed by the Exec-PHP plugin.
This works fine. However, I dislike the way this looks, with the query string. I’d much rather be able to access this page like so:
<domain>/Page_name/personal name

and have .htaccess rewrite it to the real URL.
This is something widely described on the internet, and also described in the Redirection plugin. However, I can’t get it to work.

here is how I configured it:
RewriteRule ^othername/(.*)$ Page_name/?name=$1 [L]

I’m starting this with “othername” because when I used the real name ‘Page_name’ both in the condition and the action, I got a 500 server error.
When this is configured like so however, I keep ending up on the 404 error page. I added some debug code to the 404.php page, and I can see there that the URI is still ‘/othername/person name’ so it wasn’t translated at all.

Does anybody understand what is going on here? Does WordPress do some tricks that interfere here?