Discovering 404s can be useful.

Is the old page deleted? Why not 301 redirect the URL to an appropriate page elsewhere on your site? Tools such as Screaming Frog's SEO Spider can crawl your website and help you discover 404s. By redirecting the page with a permanent redirect search engines will to pass any link juice the previous page had to the new page. Redirecting will also cleanup your pages in the SERPs and help with any broken internal links on your site (though it'd be better to fix those).

There's no need to having a rel=canonical tag on a 404 page (but if you do, ensure the tag is for the page itself and not actual content on your site).

There's also no need for search engines to index your 404 page, so I suggest adding the meta NOINDEX tag to the page.

Are there any useful keywords in your site's name? If so then definitely keep it in.

It's important to keep post titles eye-catching, which usually means short with a simple message or idea, which may leave space for your brand. Personally if I can see the brand in the title as it displays on the SERP then I'm probably more likely to click on it.

Oh and Yoast's Wordpress SEO plugin is all you need for WP...

If you have a list of sites you are going to submit to then using an extension like Roboform or AutoFillForms can take a lot of the tedium out of the process and save you a few £/$/etc if you need to do that.