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Sorry, I wrote it wrong. I meant I only have access to WP. I can edit .htaccess through Yoast plugin. I don't have cpanel access.

Yes, it should be just .com in the "WordPress Address (URL)". When I changed that, site went down and couldn't fix that without access to cpanel. I can't answer your questions because I can't reach it through FTP or some other way. "Site address (URL)" is normal .com

That's why I am so cautious about .htaccess and those redirects, because for some reasons (which I don't know) site owner doesn't provide me access to cpanel. When site went down due to that "WordPress Address (URL)" change from .com/site to .com owner didn't like it and asked that it wouldn't happen again.

I am doing SEO on WP site. Due to some duplicate pages (rel canonical was done before) I am doing 301 redirects at the moment. And I wonder if I need to redirect both links w/ and w/o trailing slash. Default is non www, w/o trailing slash.

Like there is .com/category/news but there is same page linked in .com/news (well it works when permalink structure is set to /%category%/%postname% and returns 404 error when structure is set to /%postname%). 
I redirected .lt/naujienos to .lt/category/naujienos. Should I also redirect .lt/naujienos/ (with trailing slash)?

There's absolutely no problem redirecting this, but there are some more pages which I want to edit their URLs and I wonder If I should do both redirects from links /w and w/o slash?