You can Google that question and be overwhelmed by the answers out there

Here's an interesting one

http://coschedule.com/blog/best-time-to-publish-a-blog-post/

Good question. If your website appeal is global or even national, there will always be people browsing. I agree that the niche may matter. You could try a survey. Google Forms or Survey Monkey makes that easy. Is there an app that could record the time of day you get most traffic?

I'd pick Fridays going into the weekend when people are in their email catch-up mode.

That is something you would have to test in your niche. Sometimes I have found that sending out an email on certain days is more successful. However I've never noticed it with content! Not that that means it's not there I would say test

It happens sometimes. A CTRL+F5 might help but a simple browser restart should fix it. Let me know if that helped.

You'd be able to do this in the style.css code but everytime your theme updates you'd have to redo it. I'm not a coder but if I want code for something I google the question.

I am not sure if you can change font size but you can change font if your theme supports it, to to dashboard, then customize and then you will find fonts , click on it and change as u like