Yes that's correct, or even on the same hosting account

Yes.

you don't need both sets of files.

before the request reaches your pages, it is intercepted by the webserver and checks for any 301 rules. if it finds one for that url it will redirect, even if neither of the files exist. this all happens early in the request life cycle.

But as I said before, make things easy for yourself, only 301 the pages that had external links

I would find any pages that's have external links, and only 301 them, as there is no use 301'ing pages that have no external links, you can 301 any url you want, the language is not relevant.

What server are you on IIS(Microsoft) or Apache? For Apache then use .htaccess for iis use web.config

Me myself I would not be moving to WordPress, you will end up with many more crawling problems

Having a canonical link pointing to that same url as in the address bar has no affect as far as search engines are concern, the reason moz.com gives for doing this is that if some one scrapes your site, the canonical will point back to the original.

The whole idea of canonical tags and 301's is to do with requests, you want the all requests showing the same content to appear the same page to the search engine.

With normal pages a slash means a different request that without, and to fix it you need to create a 301 that requests again to the correct url. in the process you have lost a bit of link juice.

but when requesting the home page with or without the "/", the request is the same. there is no need to fix it.

press F12 in your browser and test it yourself using the network tab, you can see that entering the url with or without the "/" on the homepage results in the same request.

If you have a trailing slash, on a url like domain.com/mypage/ then that is a different url to domain.com/mypage

If you fix this with a 301 you lose a bit of link juice in the redirect.

but if you are talking about a homepage url such as domain.com and domain.com/ these are not treated as different urls, there is no redirect between them. there is no problem here, don't worry about it

does the root domain link to the subdomain?

if so then noindex,follow the pages in the subdomain, this will allow link juice to flow back out of the subdomain.
any links pointing to a  nofollow page will waste their link juice

Unless those subdomains for single page sites, may look spammy to google. you can put those pages in your own site, there is nothing to gain using subdomains