People do it to stop scarpers, but if your going to write screen scraper it would not be hard to remove canonical tags as well. so I don't think much of the idea.

Bing recommends that you do not use self ref canonicals tags. It could be that a self ref canonical tag may be followed as is alluded to by Bing, meaning that lose a bit of link juice thought the redirect.

There is no ideal length, its more like writing a play, its not the length of the play that makes it a success, its how well you portray the story, how you set the stage how you identify the charatures

Because google will drop that url and crawl the new one by itself.

If all you are doing is changing domain then one 301 will do all anyhow.

but if you are 301'ing page by page, then just do the ones that have external links.

You can 301 any page to anywhere, that not a problem, but before you waste any time, only 301 the pages that have external links, there is no value in 301'ing pages unless they do.

That means no body clicks on them, but how did google find them? This is not evidence there is no links, just that no one has visited your site thought them

These pages may have links from other spam sites, you don't want them to return a 200.
You want them to 404, in joomla you can make the site use htaccess or not, make sure it dose and 404 the pages there.

why are they redirecting back to home page? do you redirect them or are you still infected?

I would make sure they 404