Do I need to go back and add the forward slash and should it simply be .html/" ?
No, not at all -- You just use the "main" URL for your content in the href part, so what you have is fine, with one exception: You need the protocol [http://] at the beginning, so it's an absolute URL.
Are you positive you're not thinking of xhtml grammar, where every element has to be closed? Then it would be <link rel="canonical" href="http://www.example.com/product_large_blue_widgets.html" /> but in html there's no slash.
HTML 4 is fine. You can start adding HTML 5 pieces as you go along; you don't have to change in one fell swoop. But if anyone says you should have been using XHTML for the past ten years, just put your fingers in your ears and hum loudly. Works for me :)
And yes, if you don't have full control of your own htaccess file it is time to change hosts.
I am not positive about anything right now. My site has taken a huge nose dive in the rankings and it was also hacked … for a second time ... despite doing everything the host told me to do security wise.
One of the very kind members here is helping me, but he is very busy, so am struggling along as best I can and reading as much as I can in between receiving his advice. It's a slow process.
I have made a lot of changes to the site and am updating as I go along. Crossing my fingers that all this works! :)