More .htaccess Trouble

Last week I got some help on this forum, allowing successful redirects from
domain.com/year/month/post-name/
to
domain.com/blog/year/month/post-name/

I thought that solved my problem. But now I’ve learned that external links with .html extension result in 404’s.
domain.com//year/month/post-name.html → 404, NOT FOUND.

So is there a way to make sure that all three URL variants are redirected properly? Specifically…

domain.com/year/month/post-name/
domain.com/blog/year/month/post-name/
domain.com/year/month/post-name.HTML
…should redirect to…
domain.com/blog/year/month/postname.html

My current .htaccess:

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

# END WordPress

And yes, I’ve tried resetting permalinks to default and back. That trick worked for last week’s problem, but it’s not working for this.