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I think the Yoast SEO Plugin assigns 301 redirects to existing pages, so it can't keep the redirect if you delete the old page. It sounds like you need to either add the 301 redirects directly to your .htaccess file (which isn't too tough if you only have a few), or use another plugin for redirects. The plugin called "Redirection" by John Godley does a good job.  Good luck!

For wordpress.com you have to purchase the "Site Redirect" service.  This does a 301 redirect for you entire url.  You can go to Store > Domains in your dashboard to confirm it's working, change the new domain's url, etc.  Be sure to use the same permalink structure in your new domain!  Good luck.

Agreed.  This also would have an effect on sitelinks displayed.  Focused keyword navigation leads to better sitelinks.

The basic idea for 301 redirects in htacess is:

`Redirect 301 /oldpage.html http://www.yoursite.com/newpage.html` 

But if you aren't comfortable writing out a line for each page you've deleted and need redirecting, I would suggest the "Redirection" plugin. It's pretty easy to set up.

I wouldn't recommend automatically generating the meta description because it won't necessarily be the best summary to display in the serps.  Writing something specific for users will have a better chance of click throughs.