Anything you type after your domain which isn't an actual page will return a not found error; it doesn't mean the page exists somewhere. [Try entering yourdomain.com/anythingyouwant and you will get a 404.] Or am I misunderstanding the question? In any case, 404 errors are not necessarily bad for SEO, as long as they are not harming the user experience.

Yes, once you get back into doing it everything will come back to you quickly. Have fun!

Title is not a meta tag, as for the rest, order does not matter, they have to be somewhere in the head section. The id attribute can be used on any element in HTML5 and moving it to the end will make no difference. However in general it is best to keep a standard, consistent style. Why stir things up unnecessarily?

I actually ended up installing it on my home computer as well as my work computer--I find that I have come to rely on it as a quick-and-dirty gauge of the quality of the website I am visiting...

In your chrome browser, there is a little "hamburger menu" on the top right (three horizontal lines, stacked).

Click that, go to tools > extensions and you should be able to delete it there.

ps- What do you find that it covers up?

I had worried about that too, but Matt Cutts says no, within reason. If it is a clear "read more" with a paragraph or two dropping down, that's within normal use. If you have several pages dropping down on a minimalist page, that's probably bad.

It is an old video (2011) but I haven't heard anything more recently and Google hasn't taken it down...

I am not location-specific in my PPC so I cannot speak from experience but if I had that question I would probably set up two campaigns, one UK-wide with localized keywords and the other region-specific with the more general keywords, let them run a while, and see which one performs better.

As strict as Google can be, I have never heard of them penalizing  anyone for something bad they used to do (but didn't get caught at) and have now stopped doing. I think they reserve their penalties for current bad actions. [Which is not to say you were doing something bad--we are talking about your wanting to be sure that it doesn't appear to Google that you tried something bad.]

In general, I think it's best not to disavow unless it is an extreme situation. As Google itself says: "In most cases, Google can assess which links to trust without additional guidance, so most normal or typical sites will not need to use this tool."

Since the links are using 302 redirects they are not passing any link juice, so I can't imagine that you'd be penalized for them.