This doesn't answer your questions, but one way to go would be to create assets_url and assets_path Context settings for each context that included the /en/.
Removing the /en/ from the URL is easy enough with an .htaccess rewrite, but it might interfere with your multi-language routing, your existing resource URIs, or any frozen URIs. It's worth a try, though.
RewriteRule ^(?:en|de)/(.*)$ /$1 [L]
I'm not familiar enough with multi-language, multi-context sites to suggest how to do it without messing up the routing. Maybe some one else will chime in.